diff --git a/iron-validatable-behavior.html b/iron-validatable-behavior.html
index 1b9beba..803731b 100644
--- a/iron-validatable-behavior.html
+++ b/iron-validatable-behavior.html
@@ -81,6 +81,7 @@
       },
 
       _validator: {
+        type: Object,
         computed: '__computeValidator(validator)'
       }
     },