diff --git a/components/script/dom/testbinding.rs b/components/script/dom/testbinding.rs
index e7d4d1a9cff..6dfe66a9d75 100644
--- a/components/script/dom/testbinding.rs
+++ b/components/script/dom/testbinding.rs
@@ -347,11 +347,15 @@ impl TestBindingMethods for TestBinding {
             unsignedLongValue: None,
             unsignedShortValue: None,
             usvstringValue: None,
+            nonRequiredNullable: None,
+            nonRequiredNullable2: Some(None), // null
         }
     }
 
-    fn TypeKeywordIsSuccess(&self, arg: &TestDictionary) -> bool {
-        arg.type_.as_ref().map(|s| s == "success").unwrap_or(false)
+    fn DictMatchesPassedValues(&self, arg: &TestDictionary) -> bool {
+        arg.type_.as_ref().map(|s| s == "success").unwrap_or(false) &&
+            arg.nonRequiredNullable.is_none() &&
+            arg.nonRequiredNullable2 == Some(None)
     }
 
     fn PassBoolean(&self, _: bool) {}
diff --git a/components/script/dom/webidls/TestBinding.webidl b/components/script/dom/webidls/TestBinding.webidl
index e9d94509473..6706ccfdaa9 100644
--- a/components/script/dom/webidls/TestBinding.webidl
+++ b/components/script/dom/webidls/TestBinding.webidl
@@ -32,6 +32,11 @@ dictionary TestDictionary {
   sequence<TestDictionaryDefaults> seqDict;
   // Reserved rust keyword
   DOMString type;
+  // These are used to test bidirectional conversion
+  // and differentiation of non-required and nullable types
+  // in dictionaries.
+  DOMString? nonRequiredNullable;
+  DOMString? nonRequiredNullable2;
 };
 
 dictionary TestDictionaryDefaults {
@@ -199,7 +204,7 @@ interface TestBinding {
   (unsigned long or boolean)? receiveNullableUnion5();
   sequence<long>? receiveNullableSequence();
   TestDictionary receiveTestDictionaryWithSuccessOnKeyword();
-  boolean typeKeywordIsSuccess(TestDictionary arg);
+  boolean dictMatchesPassedValues(TestDictionary arg);
 
   void passBoolean(boolean arg);
   void passByte(byte arg);
diff --git a/tests/wpt/mozilla/tests/mozilla/binding_keyword.html b/tests/wpt/mozilla/tests/mozilla/binding_keyword.html
index 2faa1ccc8f3..818d2aa2947 100644
--- a/tests/wpt/mozilla/tests/mozilla/binding_keyword.html
+++ b/tests/wpt/mozilla/tests/mozilla/binding_keyword.html
@@ -9,8 +9,9 @@ test(function() {
 
   var dict = t.receiveTestDictionaryWithSuccessOnKeyword();
   assert_equals(dict.type, "success");
+  assert_equals(dict.nonRequiredNullable, undefined);
+  assert_equals(dict.nonRequiredNullable2, null);
 
-  var is_success = t.typeKeywordIsSuccess(dict);
-  assert_true(is_success);
-}, "Conversion of dictionary elements with rust keywords works")
+  assert_true(t.dictMatchesPassedValues(dict));
+}, "Conversion of dictionary elements with rust keywords, and null non-required nullable properties works")
 </script>