PromiseCoerce should ignore primitive values.

src/promise.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 199 matching lines @@
       return x === that ? onReject(MakeTypeError('promise_cyclic', [x])) :
              IsPromise(x) ? x.then(onResolve, onReject) : onResolve(x);
     },
     onReject
   );
 }
 
 PromiseCoerce.table = new $WeakMap;
 
 function PromiseCoerce(constructor, x) {
-  if (!(IsPromise(x) || IS_NULL_OR_UNDEFINED(x))) {
+  if (!IsPromise(x) && IS_SPEC_OBJECT(x)) {
     var then = x.then;
     if (typeof then === 'function') {
       if (PromiseCoerce.table.has(x)) {
         return PromiseCoerce.table.get(x);
       } else {
         var deferred = %_CallFunction(constructor, PromiseDeferred);
         PromiseCoerce.table.set(x, deferred.promise);
         try {
           %_CallFunction(x, deferred.resolve, deferred.reject, then);
         } catch(e) {
@@ skipping 68 matching lines @@
     "cast", PromiseCast
   ]);
   InstallFunctions($Promise.prototype, DONT_ENUM, [
     "chain", PromiseChain,
     "then", PromiseThen,
     "catch", PromiseCatch
   ]);
 }
 
 SetUpPromise();