| Index: src/weak-collection.js | 
| diff --git a/src/weak-collection.js b/src/weak-collection.js | 
| index 933d753f3fab74457717952ba91dc36423ba21b4..67deddb95ee57491b585bf8d1f0cebe4be800d3d 100644 | 
| --- a/src/weak-collection.js | 
| +++ b/src/weak-collection.js | 
| @@ -25,7 +25,7 @@ function WeakMapConstructor(iterable) { | 
|  | 
| if (!IS_NULL_OR_UNDEFINED(iterable)) { | 
| var adder = this.set; | 
| -    if (!IS_SPEC_FUNCTION(adder)) { | 
| +    if (!IS_CALLABLE(adder)) { | 
| throw MakeTypeError(kPropertyNotFunction, 'set', this); | 
| } | 
| for (var nextItem of iterable) { | 
| @@ -114,7 +114,7 @@ function WeakSetConstructor(iterable) { | 
|  | 
| if (!IS_NULL_OR_UNDEFINED(iterable)) { | 
| var adder = this.add; | 
| -    if (!IS_SPEC_FUNCTION(adder)) { | 
| +    if (!IS_CALLABLE(adder)) { | 
| throw MakeTypeError(kPropertyNotFunction, 'add', this); | 
| } | 
| for (var value of iterable) { | 
|  |