diff --git a/src/lib/util.js b/src/lib/util.js
index e28d45a9..9e77a0b0 100644
--- a/src/lib/util.js
+++ b/src/lib/util.js
@@ -400,7 +400,7 @@ export let util = {
 
             if (result !== undefined) {
                 newobj[key] = result;
-            } else if (typeof item === 'object') {
+            } else if (typeof item === 'object' && item !== null) {
                 newobj[key] = util.replaceObject(item, callback);
             } else {
                 newobj[key] = item;