diff --git a/lib/models/acl.js b/lib/models/acl.js
index b14ebabc..ce6f936f 100644
--- a/lib/models/acl.js
+++ b/lib/models/acl.js
@@ -296,6 +296,9 @@ ACL.getStaticACLs = function getStaticACLs(model, property) {
 ACL.checkPermission = function checkPermission(principalType, principalId,
                                                model, property, accessType,
                                                callback) {
+  if(principalId !== null && principalId !== undefined && (typeof principalId !== 'string') ) {
+    principalId = principalId.toString();
+  }
   property = property || ACL.ALL;
   var propertyQuery = (property === ACL.ALL) ? undefined : {inq: [property, ACL.ALL]};
   accessType = accessType || ACL.ALL;