diff --git a/FBKVOController/FBKVOController.h b/FBKVOController/FBKVOController.h
index fd694c5..58fcaa9 100644
--- a/FBKVOController/FBKVOController.h
+++ b/FBKVOController/FBKVOController.h
@@ -9,14 +9,15 @@
 
 #import <Foundation/Foundation.h>
 
+NS_ASSUME_NONNULL_BEGIN
+
 /**
  @abstract Block called on key-value change notification.
  @param observer The observer of the change.
  @param object The object changed.
  @param change The change dictionary.
  */
-typedef void (^FBKVONotificationBlock)(id observer, id object, NSDictionary *change);
-
+typedef void (^FBKVONotificationBlock)(id _Nullable observer, id object, NSDictionary *change);
 
 /**
  @abstract FBKVOController makes Key-Value Observing simpler and safer.
@@ -29,7 +30,7 @@ typedef void (^FBKVONotificationBlock)(id observer, id object, NSDictionary *cha
  @param observer The object notified on key-value change.
  @return The initialized KVO controller instance.
  */
-+ (instancetype)controllerWithObserver:(id)observer;
++ (instancetype)controllerWithObserver:(nullable id)observer;
 
 /**
  @abstract The designated initializer.
@@ -38,7 +39,7 @@ typedef void (^FBKVONotificationBlock)(id observer, id object, NSDictionary *cha
  @return The initialized KVO controller instance.
  @discussion Use retainObserved = NO when a strong reference between controller and observee would create a retain loop. When not retaining observees, special care must be taken to remove observation info prior to observee dealloc.
  */
-- (instancetype)initWithObserver:(id)observer retainObserved:(BOOL)retainObserved;
+- (instancetype)initWithObserver:(nullable id)observer retainObserved:(BOOL)retainObserved;
 
 /**
  @abstract Convenience initializer.
@@ -46,10 +47,10 @@ typedef void (^FBKVONotificationBlock)(id observer, id object, NSDictionary *cha
  @return The initialized KVO controller instance.
  @discussion By default, KVO controller retains objects observed.
  */
-- (instancetype)initWithObserver:(id)observer;
+- (instancetype)initWithObserver:(nullable id)observer;
 
 /// The observer notified on key-value change. Specified on initialization.
-@property (atomic, weak, readonly) id observer;
+@property (nullable, atomic, weak, readonly) id observer;
 
 /**
  @abstract Registers observer for key-value change notification.
@@ -59,7 +60,7 @@ typedef void (^FBKVONotificationBlock)(id observer, id object, NSDictionary *cha
  @param block The block to execute on notification.
  @discussion On key-value change, the specified block is called. In order to avoid retain loops, the block must avoid referencing the KVO controller or an owner thereof. Observing an already observed object key path or nil results in no operation.
  */
-- (void)observe:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options block:(FBKVONotificationBlock)block;
+- (void)observe:(nullable id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options block:(FBKVONotificationBlock)block;
 
 /**
  @abstract Registers observer for key-value change notification.
@@ -69,7 +70,7 @@ typedef void (^FBKVONotificationBlock)(id observer, id object, NSDictionary *cha
  @param action The observer selector called on key-value change.
  @discussion On key-value change, the observer's action selector is called. The selector provided should take the form of -propertyDidChange, -propertyDidChange: or -propertyDidChange:object:, where optional parameters delivered will be KVO change dictionary and object observed. Observing nil or observing an already observed object's key path results in no operation.
  */
-- (void)observe:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options action:(SEL)action;
+- (void)observe:(nullable id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options action:(SEL)action;
 
 /**
  @abstract Registers observer for key-value change notification.
@@ -79,7 +80,7 @@ typedef void (^FBKVONotificationBlock)(id observer, id object, NSDictionary *cha
  @param context The context specified.
  @discussion On key-value change, the observer's -observeValueForKeyPath:ofObject:change:context: method is called. Observing an already observed object key path or nil results in no operation.
  */
-- (void)observe:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context;
+- (void)observe:(nullable id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context;
 
 
 /**
@@ -90,7 +91,7 @@ typedef void (^FBKVONotificationBlock)(id observer, id object, NSDictionary *cha
  @param block The block to execute on notification.
  @discussion On key-value change, the specified block is called. Inorder to avoid retain loops, the block must avoid referencing the KVO controller or an owner thereof. Observing an already observed object key path or nil results in no operation.
  */
-- (void)observe:(id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObservingOptions)options block:(FBKVONotificationBlock)block;
+- (void)observe:(nullable id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObservingOptions)options block:(FBKVONotificationBlock)block;
 
 /**
  @abstract Registers observer for key-value change notification.
@@ -100,7 +101,7 @@ typedef void (^FBKVONotificationBlock)(id observer, id object, NSDictionary *cha
  @param action The observer selector called on key-value change.
  @discussion On key-value change, the observer's action selector is called. The selector provided should take the form of -propertyDidChange, -propertyDidChange: or -propertyDidChange:object:, where optional parameters delivered will be KVO change dictionary and object observed. Observing nil or observing an already observed object's key path results in no operation.
  */
-- (void)observe:(id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObservingOptions)options action:(SEL)action;
+- (void)observe:(nullable id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObservingOptions)options action:(SEL)action;
 
 /**
  @abstract Registers observer for key-value change notification.
@@ -110,7 +111,7 @@ typedef void (^FBKVONotificationBlock)(id observer, id object, NSDictionary *cha
  @param context The context specified.
  @discussion On key-value change, the observer's -observeValueForKeyPath:ofObject:change:context: method is called. Observing an already observed object key path or nil results in no operation.
  */
-- (void)observe:(id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObservingOptions)options context:(void *)context;
+- (void)observe:(nullable id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObservingOptions)options context:(nullable void *)context;
 
 
 /**
@@ -119,14 +120,14 @@ typedef void (^FBKVONotificationBlock)(id observer, id object, NSDictionary *cha
  @param keyPath The key path to observe.
  @discussion If not observing object key path, or unobserving nil, this method results in no operation.
  */
-- (void)unobserve:(id)object keyPath:(NSString *)keyPath;
+- (void)unobserve:(nullable id)object keyPath:(NSString *)keyPath;
 
 /**
  @abstract Unobserve all object key paths.
  @param object The object to unobserve.
  @discussion If not observing object, or unobserving nil, this method results in no operation.
  */
-- (void)unobserve:(id)object;
+- (void)unobserve:(nullable id)object;
 
 /**
  @abstract Unobserve all objects.
@@ -135,3 +136,5 @@ typedef void (^FBKVONotificationBlock)(id observer, id object, NSDictionary *cha
 - (void)unobserveAll;
 
 @end
+
+NS_ASSUME_NONNULL_END
diff --git a/FBKVOController/FBKVOController.m b/FBKVOController/FBKVOController.m
index e1c46c0..33f32e8 100644
--- a/FBKVOController/FBKVOController.m
+++ b/FBKVOController/FBKVOController.m
@@ -16,6 +16,8 @@
 #error This file must be compiled with ARC. Convert your project to ARC or specify the -fobjc-arc flag.
 #endif
 
+NS_ASSUME_NONNULL_BEGIN
+
 #pragma mark Utilities -
 
 static NSString *describe_option(NSKeyValueObservingOptions option)
@@ -98,7 +100,12 @@ @implementation _FBKVOInfo
   FBKVONotificationBlock _block;
 }
 
-- (instancetype)initWithController:(FBKVOController *)controller keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options block:(FBKVONotificationBlock)block action:(SEL)action context:(void *)context
+- (instancetype)initWithController:(FBKVOController *)controller
+                           keyPath:(NSString *)keyPath
+                           options:(NSKeyValueObservingOptions)options
+                             block:(nullable FBKVONotificationBlock)block
+                            action:(nullable SEL)action
+                           context:(nullable void *)context
 {
   self = [super init];
   if (nil != self) {
@@ -184,13 +191,13 @@ @interface _FBKVOSharedController : NSObject
 + (instancetype)sharedController;
 
 /** observe an object, info pair */
-- (void)observe:(id)object info:(_FBKVOInfo *)info;
+- (void)observe:(id)object info:(nullable _FBKVOInfo *)info;
 
 /** unobserve an object, info pair */
-- (void)unobserve:(id)object info:(_FBKVOInfo *)info;
+- (void)unobserve:(id)object info:(nullable _FBKVOInfo *)info;
 
 /** unobserve an object with a set of infos */
-- (void)unobserve:(id)object infos:(NSSet *)infos;
+- (void)unobserve:(id)object infos:(nullable NSSet *)infos;
 
 @end
 
@@ -255,7 +262,7 @@ - (NSString *)debugDescription
   return s;
 }
 
-- (void)observe:(id)object info:(_FBKVOInfo *)info
+- (void)observe:(id)object info:(nullable _FBKVOInfo *)info
 {
   if (nil == info) {
     return;
@@ -270,7 +277,7 @@ - (void)observe:(id)object info:(_FBKVOInfo *)info
   [object addObserver:self forKeyPath:info->_keyPath options:info->_options context:(void *)info];
 }
 
-- (void)unobserve:(id)object info:(_FBKVOInfo *)info
+- (void)unobserve:(id)object info:(nullable _FBKVOInfo *)info
 {
   if (nil == info) {
     return;
@@ -285,7 +292,7 @@ - (void)unobserve:(id)object info:(_FBKVOInfo *)info
   [object removeObserver:self forKeyPath:info->_keyPath context:(void *)info];
 }
 
-- (void)unobserve:(id)object infos:(NSSet *)infos
+- (void)unobserve:(id)object infos:(nullable NSSet *)infos
 {
   if (0 == infos.count) {
     return;
@@ -304,7 +311,10 @@ - (void)unobserve:(id)object infos:(NSSet *)infos
   }
 }
 
-- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
+- (void)observeValueForKeyPath:(nullable NSString *)keyPath
+                      ofObject:(nullable id)object
+                        change:(nullable NSDictionary *)change
+                       context:(nullable void *)context
 {
   NSAssert(context, @"missing context keyPath:%@ object:%@ change:%@", keyPath, object, change);
   
@@ -355,12 +365,12 @@ @implementation FBKVOController
 
 #pragma mark Lifecycle -
 
-+ (instancetype)controllerWithObserver:(id)observer
++ (instancetype)controllerWithObserver:(nullable id)observer
 {
   return [[self alloc] initWithObserver:observer];
 }
 
-- (instancetype)initWithObserver:(id)observer retainObserved:(BOOL)retainObserved
+- (instancetype)initWithObserver:(nullable id)observer retainObserved:(BOOL)retainObserved
 {
   self = [super init];
   if (nil != self) {
@@ -372,7 +382,7 @@ - (instancetype)initWithObserver:(id)observer retainObserved:(BOOL)retainObserve
   return self;
 }
 
-- (instancetype)initWithObserver:(id)observer
+- (instancetype)initWithObserver:(nullable id)observer
 {
   return [self initWithObserver:observer retainObserved:YES];
 }
@@ -514,7 +524,7 @@ - (void)_unobserveAll
 
 #pragma mark API -
 
-- (void)observe:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options block:(FBKVONotificationBlock)block
+- (void)observe:(nullable id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options block:(FBKVONotificationBlock)block
 {
   NSAssert(0 != keyPath.length && NULL != block, @"missing required parameters observe:%@ keyPath:%@ block:%p", object, keyPath, block);
   if (nil == object || 0 == keyPath.length || NULL == block) {
@@ -529,7 +539,7 @@ - (void)observe:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObserv
 }
 
 
-- (void)observe:(id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObservingOptions)options block:(FBKVONotificationBlock)block
+- (void)observe:(nullable id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObservingOptions)options block:(FBKVONotificationBlock)block
 {
   NSAssert(0 != keyPaths.count && NULL != block, @"missing required parameters observe:%@ keyPath:%@ block:%p", object, keyPaths, block);
   if (nil == object || 0 == keyPaths.count || NULL == block) {
@@ -542,7 +552,7 @@ - (void)observe:(id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObser
   }
 }
 
-- (void)observe:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options action:(SEL)action
+- (void)observe:(nullable id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options action:(SEL)action
 {
   NSAssert(0 != keyPath.length && NULL != action, @"missing required parameters observe:%@ keyPath:%@ action:%@", object, keyPath, NSStringFromSelector(action));
   NSAssert([_observer respondsToSelector:action], @"%@ does not respond to %@", _observer, NSStringFromSelector(action));
@@ -557,7 +567,7 @@ - (void)observe:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObserv
   [self _observe:object info:info];
 }
 
-- (void)observe:(id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObservingOptions)options action:(SEL)action
+- (void)observe:(nullable id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObservingOptions)options action:(SEL)action
 {
   NSAssert(0 != keyPaths.count && NULL != action, @"missing required parameters observe:%@ keyPath:%@ action:%@", object, keyPaths, NSStringFromSelector(action));
   NSAssert([_observer respondsToSelector:action], @"%@ does not respond to %@", _observer, NSStringFromSelector(action));
@@ -571,7 +581,7 @@ - (void)observe:(id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObser
   }
 }
 
-- (void)observe:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context
+- (void)observe:(nullable id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context
 {
   NSAssert(0 != keyPath.length, @"missing required parameters observe:%@ keyPath:%@", object, keyPath);
   if (nil == object || 0 == keyPath.length) {
@@ -585,7 +595,7 @@ - (void)observe:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObserv
   [self _observe:object info:info];
 }
 
-- (void)observe:(id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObservingOptions)options context:(void *)context
+- (void)observe:(nullable id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObservingOptions)options context:(nullable void *)context
 {
   NSAssert(0 != keyPaths.count, @"missing required parameters observe:%@ keyPath:%@", object, keyPaths);
   if (nil == object || 0 == keyPaths.count) {
@@ -598,7 +608,7 @@ - (void)observe:(id)object keyPaths:(NSArray *)keyPaths options:(NSKeyValueObser
   }
 }
 
-- (void)unobserve:(id)object keyPath:(NSString *)keyPath
+- (void)unobserve:(nullable id)object keyPath:(NSString *)keyPath
 {
   // create representative info
   _FBKVOInfo *info = [[_FBKVOInfo alloc] initWithController:self keyPath:keyPath];
@@ -607,7 +617,7 @@ - (void)unobserve:(id)object keyPath:(NSString *)keyPath
   [self _unobserve:object info:info];
 }
 
-- (void)unobserve:(id)object
+- (void)unobserve:(nullable id)object
 {
   if (nil == object) {
     return;
@@ -622,3 +632,5 @@ - (void)unobserveAll
 }
 
 @end
+
+NS_ASSUME_NONNULL_END
diff --git a/FBKVOController/NSObject+FBKVOController.h b/FBKVOController/NSObject+FBKVOController.h
index 71220a0..a0cdab4 100644
--- a/FBKVOController/NSObject+FBKVOController.h
+++ b/FBKVOController/NSObject+FBKVOController.h
@@ -8,8 +8,11 @@
  */
 
 #import <Foundation/Foundation.h>
+
 #import "FBKVOController.h"
 
+NS_ASSUME_NONNULL_BEGIN
+
 @interface NSObject (FBKVOController)
 
 /**
@@ -21,3 +24,5 @@
 @property (nonatomic, strong) FBKVOController *KVOControllerNonRetaining;
 
 @end
+
+NS_ASSUME_NONNULL_END
diff --git a/FBKVOController/NSObject+FBKVOController.m b/FBKVOController/NSObject+FBKVOController.m
index ddc8e56..4f39ace 100644
--- a/FBKVOController/NSObject+FBKVOController.m
+++ b/FBKVOController/NSObject+FBKVOController.m
@@ -18,6 +18,8 @@
 
 #pragma mark NSObject Category -
 
+NS_ASSUME_NONNULL_BEGIN
+
 static void *NSObjectKVOControllerKey = &NSObjectKVOControllerKey;
 static void *NSObjectKVOControllerNonRetainingKey = &NSObjectKVOControllerNonRetainingKey;
 
@@ -60,3 +62,5 @@ - (void)setKVOControllerNonRetaining:(FBKVOController *)KVOControllerNonRetainin
 
 @end
 
+
+NS_ASSUME_NONNULL_END
diff --git a/FBKVOControllerTests/FBKVOControllerTests.m b/FBKVOControllerTests/FBKVOControllerTests.m
index 9d15cb2..becedfa 100644
--- a/FBKVOControllerTests/FBKVOControllerTests.m
+++ b/FBKVOControllerTests/FBKVOControllerTests.m
@@ -155,7 +155,7 @@ - (void)testObserveKeyPathsOptionsBlockWhenKeyPathsIsNilRaises
 {
   FBKVOController *controller = [FBKVOController controllerWithObserver:nil];
   FBKVONotificationBlock arbitraryBlock = ^(id observer, id object, NSDictionary *change) { /* noop */ };
-  XCTAssertThrows([controller observe:nil keyPaths:nil options:0 block:arbitraryBlock]);
+  XCTAssertThrows([controller observe:nil keyPaths:(id _Nonnull)nil options:0 block:arbitraryBlock]);
 }
 
 - (void)testObserveKeyPathsOptionsBlockWhenKeyPathsIsEmptyRaises
@@ -170,7 +170,7 @@ - (void)testObserveKeyPathsOptionsBlockWhenBlockIsNilRaises
 {
   FBKVOController *controller = [FBKVOController controllerWithObserver:nil];
   NSArray *arbitraryKeyPaths = @[@"ante", @"bellum"];
-  XCTAssertThrows([controller observe:nil keyPaths:arbitraryKeyPaths options:0 block:nil]);
+  XCTAssertThrows([controller observe:nil keyPaths:arbitraryKeyPaths options:0 block:(id _Nonnull)nil]);
 }
 
 - (void)testObserveKeyPathsOptionsBlockWhenObjectIsNilDoesNothing
@@ -207,7 +207,7 @@ - (void)testObserveKeyPathsOptionsActionWhenKeyPathsIsNilRaises
 {
   FBKVOController *controller = [FBKVOController controllerWithObserver:nil];
   SEL arbitrarySelector = @selector(cookies);
-  XCTAssertThrows([controller observe:nil keyPaths:nil options:0 action:arbitrarySelector]);
+  XCTAssertThrows([controller observe:nil keyPaths:(id _Nonnull)nil options:0 action:arbitrarySelector]);
 }
 
 - (void)testObserveKeyPathsOptionsActionWhenKeyPathsIsEmptyRaises
@@ -222,7 +222,7 @@ - (void)testObserveKeyPathsOptionsActionWhenActionIsNullRaises
 {
   FBKVOController *controller = [FBKVOController controllerWithObserver:nil];
   NSArray *arbitraryKeyPaths = @[@"carpe", @"diem"];
-  XCTAssertThrows([controller observe:nil keyPaths:arbitraryKeyPaths options:0 action:NULL]);
+  XCTAssertThrows([controller observe:nil keyPaths:arbitraryKeyPaths options:0 action:(SEL _Nonnull)NULL]);
 }
 
 - (void)testObserveKeyPathsOptionsActionWhenObjectIsNilRaises
@@ -263,7 +263,7 @@ - (void)testObserveKeyPathsOptionsActionObservesEachOfTheKeyPaths
 - (void)testObserveKeyPathsOptionsContextWhenKeyPathsIsNilRaises
 {
   FBKVOController *controller = [FBKVOController controllerWithObserver:nil];
-  XCTAssertThrows([controller observe:nil keyPaths:nil options:0 context:NULL]);
+  XCTAssertThrows([controller observe:nil keyPaths:(id _Nonnull)nil options:0 context:NULL]);
 }
 
 - (void)testObserveKeyPathsOptionsContextWhenKeyPathsIsEmptyRaises