diff --git a/src/Foundation/NSObject2.cs b/src/Foundation/NSObject2.cs
index c1df9c2b5c24..3f8f412260e9 100644
--- a/src/Foundation/NSObject2.cs
+++ b/src/Foundation/NSObject2.cs
@@ -828,6 +828,9 @@ public IDisposable AddObserver (string key, NSKeyValueObservingOptions options,
 
 		public IDisposable AddObserver (NSString key, NSKeyValueObservingOptions options, Action<NSObservedChange> observer)
 		{
+			if (observer == null)
+				throw new ArgumentNullException ("observer");
+
 			var o = new Observer (this, key, observer);
 			AddObserver (o, key, options, o.Handle);
 			return o;