diff --git a/spec/operators/zipAll-spec.ts b/spec/operators/zipAll-spec.ts
index 376c1fb469..6b8d0af1c1 100644
--- a/spec/operators/zipAll-spec.ts
+++ b/spec/operators/zipAll-spec.ts
@@ -39,7 +39,7 @@ describe('zipAll operator', () => {
       of('a', 'b', 'c'),
       of(1, 2, 3)
     )
-    .pipe(zipAll((a, b) => a + b))
+    .pipe(zipAll((a, b) => String(a) + String(b)))
     .subscribe((x) => {
       expect(x).to.equal(expected[i++]);
     }, null, done);
@@ -231,7 +231,7 @@ describe('zipAll operator', () => {
       const b = [4, 5, 6];
       const expected = '---x--#';
 
-      const selector = function (x: number, y: number) {
+      const selector = function (x: string, y: number) {
         if (y === 5) {
           throw new Error('too bad');
         } else {
@@ -378,7 +378,7 @@ describe('zipAll operator', () => {
       of('a', 'b', 'c'),
       of(1, 2)
     )
-    .pipe(zipAll((a, b) => a + b))
+    .pipe(zipAll((a, b) => String(a) + String(b)))
     .subscribe((x) => {
       expect(x).to.equal(expected[i++]);
     }, null, done);
diff --git a/src/internal/types.ts b/src/internal/types.ts
index f29cae4194..9581f98ccb 100644
--- a/src/internal/types.ts
+++ b/src/internal/types.ts
@@ -39,9 +39,8 @@ export type SubscribableOrPromise<T> = Subscribable<T> | Subscribable<never> | P
 /** OBSERVABLE INTERFACES */
 
 export interface Subscribable<T> {
-  subscribe(observerOrNext?: PartialObserver<T> | ((value: T) => void),
-            error?: (error: any) => void,
-            complete?: () => void): Unsubscribable;
+  subscribe(observer?: PartialObserver<T>): Unsubscribable;
+  subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): Unsubscribable;
 }
 
 export type ObservableInput<T> = SubscribableOrPromise<T> | ArrayLike<T> | Iterable<T>;