diff --git a/harness/isConstructor.js b/harness/isConstructor.js
index 5cad484242..b23001c5e5 100644
--- a/harness/isConstructor.js
+++ b/harness/isConstructor.js
@@ -8,6 +8,10 @@ defines: [isConstructor]
 ---*/
 
 function isConstructor(f) {
+    if (typeof f !== "function") {
+      throw new Test262Error("isConstructor invoked with a non-function value");
+    }
+
     try {
         Reflect.construct(function(){}, [], f);
     } catch (e) {
diff --git a/test/built-ins/Promise/prototype/finally/rejected-observable-then-calls-argument.js b/test/built-ins/Promise/prototype/finally/rejected-observable-then-calls-argument.js
index 43306e8aca..76ba16dfaf 100644
--- a/test/built-ins/Promise/prototype/finally/rejected-observable-then-calls-argument.js
+++ b/test/built-ins/Promise/prototype/finally/rejected-observable-then-calls-argument.js
@@ -40,10 +40,6 @@ var expected = [
 
 var then = Promise.prototype.then;
 Promise.prototype.then = function(resolve, reject) {
-  assert.sameValue(isConstructor(reject), false, 'isConstructor(reject) must return false');
-  assert.throws(TypeError, () => {
-    new reject();
-  }, '`new reject()` throws TypeError');
 
   assert.sameValue(
     resolve.length,
@@ -57,6 +53,13 @@ Promise.prototype.then = function(resolve, reject) {
   );
   if (calls === 0) {
     assert.throws(MyError, resolve, '`resolve()` throws `MyError`');
+    assert.sameValue(arguments.length, 1, '`then` invoked with one argument');
+  } else {
+    assert.sameValue(isConstructor(reject), false, 'isConstructor(reject) must return false');
+    assert.throws(TypeError, () => {
+      new reject();
+    }, '`new reject()` throws TypeError');
+    assert.sameValue(arguments.length, 2, '`then` invoked with two arguments');
   }
 
   calls += 1;
diff --git a/test/harness/isConstructor.js b/test/harness/isConstructor.js
index 54460ce149..49504996ca 100644
--- a/test/harness/isConstructor.js
+++ b/test/harness/isConstructor.js
@@ -13,16 +13,16 @@ features: [generators, Reflect.construct]
 
 assert.sameValue(typeof isConstructor, "function");
 
-assert.sameValue(isConstructor(), false);
-assert.sameValue(isConstructor(undefined), false);
-assert.sameValue(isConstructor(null), false);
-assert.sameValue(isConstructor(123), false);
-assert.sameValue(isConstructor(true), false);
-assert.sameValue(isConstructor(false), false);
-assert.sameValue(isConstructor("string"), false);
+assert.throws(Test262Error, () => isConstructor(), "no argument");
+assert.throws(Test262Error, () => isConstructor(undefined), "undefined");
+assert.throws(Test262Error, () => isConstructor(null), "null");
+assert.throws(Test262Error, () => isConstructor(123), "number");
+assert.throws(Test262Error, () => isConstructor(true), "boolean - true");
+assert.throws(Test262Error, () => isConstructor(false), "boolean - false");
+assert.throws(Test262Error, () => isConstructor("string"), "string");
 
-assert.sameValue(isConstructor({}), false);
-assert.sameValue(isConstructor([]), false);
+assert.throws(Test262Error, () => isConstructor({}), "Object instance");
+assert.throws(Test262Error, () => isConstructor([]), "Array instance");
 
 assert.sameValue(isConstructor(function(){}), true);
 assert.sameValue(isConstructor(function*(){}), false);