diff --git a/src/ng/directive/ngModel.js b/src/ng/directive/ngModel.js
index 55310adceddd..a74e8a4bffae 100644
--- a/src/ng/directive/ngModel.js
+++ b/src/ng/directive/ngModel.js
@@ -603,7 +603,7 @@ NgModelController.prototype = {
     function processSyncValidators() {
       var syncValidatorsValid = true;
       forEach(that.$validators, function(validator, name) {
-        var result = validator(modelValue, viewValue);
+        var result = Boolean(validator(modelValue, viewValue));
         syncValidatorsValid = syncValidatorsValid && result;
         setValidity(name, result);
       });
diff --git a/test/ng/directive/ngModelSpec.js b/test/ng/directive/ngModelSpec.js
index 37c633f13c97..b21f21baeea1 100644
--- a/test/ng/directive/ngModelSpec.js
+++ b/test/ng/directive/ngModelSpec.js
@@ -847,6 +847,32 @@ describe('ngModel', function() {
         expect(ctrl.$valid).toBe(true);
       });
 
+      it('should treat all responses as boolean for synchronous validators', function() {
+        var expectValid = function(value, expected) {
+          ctrl.$modelValue = undefined;
+          ctrl.$validators.a = valueFn(value);
+
+          ctrl.$validate();
+          expect(ctrl.$valid).toBe(expected);
+        };
+
+        // False tests
+        expectValid(false, false);
+        expectValid(undefined, false);
+        expectValid(null, false);
+        expectValid(0, false);
+        expectValid(NaN, false);
+        expectValid('', false);
+
+        // True tests
+        expectValid(true, true);
+        expectValid(1, true);
+        expectValid('0', true);
+        expectValid('false', true);
+        expectValid([], true);
+        expectValid({}, true);
+      });
+
 
       it('should register invalid validations on the $error object', function() {
         var curry = function(v) {