Index: graphspell-js/helpers.js
==================================================================
--- graphspell-js/helpers.js
+++ graphspell-js/helpers.js
@@ -72,18 +72,24 @@
         }
     },
 
     // conversions
     objectToMap: function (obj) {
+        if (obj == null) {
+            return null;
+        }
         let m = new Map();
         for (let param in obj) {
             m.set(param, obj[param]);
         }
         return m;
     },
 
     mapToObject: function (m) {
+        if (m == null) {
+            return null;
+        }
         let obj = {};
         for (let [k, v] of m) {
             obj[k] = v;
         }
         return obj;