diff --git a/lib/index.js b/lib/index.js
index 73a7cc4..3ca828e 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -157,7 +157,7 @@ function setCurrent (state, type, value) {
 function checkLocalCircular (schema) {
   const dag = new DAG()
   const locals = traverse(schema).reduce(function (acc, node) {
-    if (!_.isNull(node) && !_.isUndefined(null) && typeof node.$ref === 'string') {
+    if (!_.isNull(node) && !_.isUndefined(node) && typeof node.$ref === 'string') {
       const refType = utils.getRefType(node)
       if (refType === 'local') {
         const value = utils.getRefValue(node)
@@ -221,7 +221,7 @@ function derefSchema (schema, options, state) {
   }
 
   return traverse(schema).forEach(function (node) {
-    if (!_.isNull(node) && !_.isUndefined(null) && typeof node.$ref === 'string') {
+    if (!_.isNull(node) && !_.isUndefined(node) && typeof node.$ref === 'string') {
       const refType = utils.getRefType(node)
       const refVal = utils.getRefValue(node)