[BUGFIX beta] possible fix for #15490

packages/ember-testing/lib/test/promise.js

@@ -9,8 +9,10 @@ export default class TestPromise extends RSVP.Promise {
     lastPromise = this;
   }
 
-  then(onFulfillment, ...args) {
-    return super.then(result => isolate(onFulfillment, result), ...args);
+  then(_onFulfillment, ...args) {
+    let onFulfillment = typeof _onFulfillment === 'function' ?
+      result => isolate(_onFulfillment, result) : undefined;
+    return super.then(onFulfillment, ...args);
   }
 }
 

packages/ember-testing/tests/ext/rsvp_test.js

@@ -1,5 +1,6 @@
 import RSVP from '../../ext/rsvp';
 import { getAdapter, setAdapter } from '../../test/adapter';
+import TestPromise, { getLastPromise } from '../../test/promise';
 import { run } from 'ember-metal';
 import { isTesting, setTesting } from 'ember-debug';
 
@@ -84,3 +85,35 @@ QUnit.test('given `Ember.testing = true`, correctly informs the test suite about
     equal(asyncEnded, 2);
   });
 });
+
+
+QUnit.module('TestPromise');
+
+QUnit.test('does not throw error when falsy value passed to then', function() {
+  expect(1);
+  return new TestPromise(function(resolve) {
+    resolve()
+  })
+  .then(null)
+  .then(function() {
+    ok(true);
+  });
+});
+
+QUnit.test('able to get last Promise', function() {
+  expect(2);
+
+  var p1 = new TestPromise(function(resolve) {
+    resolve()
+  })
+  .then(function() {
+    ok(true);
+  });
+
+  var p2 = new TestPromise(function(resolve) {
+    resolve()
+  });
+
+  deepEqual(getLastPromise(), p2);
+  return p1;
+});