Merge pull request #671 from evdokim/patch-1

an additional null check added

Author: fehguy

lib/resolver.js

@@ -565,7 +565,7 @@ Resolver.prototype.resolveTo = function (root, property, resolutionTable, locati
   var sp, i;
   var ref = property.$ref;
   var lroot = root;
-  if (typeof ref !== 'undefined') {
+  if ((typeof ref !== 'undefined') && (ref !== null)) {
     if(ref.indexOf('#') >= 0) {
       var parts = ref.split('#');