diff --git a/backbone-nested-models.js b/backbone-nested-models.js
index 45f0be2..2c5a8ce 100644
--- a/backbone-nested-models.js
+++ b/backbone-nested-models.js
@@ -150,7 +150,9 @@
 
       _.each(this.relations, function(rel, key) {
         if (_.has(attrs, key)) {
-          attrs[key] = attrs[key].toJSON();
+          if(attrs[key] !== null) {
+            attrs[key] = attrs[key].toJSON();
+          }
         } else {
             attrs[key] = (new rel()).toJSON();
         }