diff --git a/Src/knockout.validation.js b/Src/knockout.validation.js
index ab1422c3..33e97d9b 100644
--- a/Src/knockout.validation.js
+++ b/Src/knockout.validation.js
@@ -617,7 +617,7 @@
     validation.rules['phoneUS'] = {
         validator: function (phoneNumber, validate) {
             if (!validate) return true;
-            if (typeof (phoneNumber) !== 'string') { return false; }
+            if (phoneNumber !== null && typeof (phoneNumber) !== 'string') { return false; }
             if (utils.isEmptyVal(phoneNumber)) { return true; } // makes it optional, use 'required' rule if it should be required
             phoneNumber = phoneNumber.replace(/\s+/g, "");
             return validate && phoneNumber.length > 9 && phoneNumber.match(/^(1-?)?(\([2-9]\d{2}\)|[2-9]\d{2})-?[2-9]\d{2}-?\d{4}$/);
diff --git a/Tests/validation-tests.js b/Tests/validation-tests.js
index 65896b6f..bf048507 100644
--- a/Tests/validation-tests.js
+++ b/Tests/validation-tests.js
@@ -572,6 +572,15 @@ test('Object is NOT Valid and isValid returns False', function () {
     equal(testObj.isValid(), false, 'testObj is not valid');
 });
 
+test('Object is null and isValid returns True', function () {
+    var testObj = ko.observable('').extend({ phoneUS: true });
+
+    testObj(null);
+
+    equal(testObj(), null, 'observable still works');
+    equal(testObj.isValid(), true, 'testObj is valid');
+});
+
 //#endregion
 
 //#region Custom Rule Validation