diff --git a/lib/dao.js b/lib/dao.js
index bd9c8180..7ffdc700 100644
--- a/lib/dao.js
+++ b/lib/dao.js
@@ -179,10 +179,12 @@ DataAccessObject._forDB = function(data) {
   const res = {};
   for (const propName in data) {
     const type = this.getPropertyType(propName);
-    if (type === 'JSON' || type === 'Any' || type === 'Object' || data[propName] instanceof Array) {
-      res[propName] = JSON.stringify(data[propName]);
+    const value = data[propName];
+    if (value !== null && (type === 'JSON' || type === 'Any' ||
+        type === 'Object' || value instanceof Array)) {
+      res[propName] = JSON.stringify(value);
     } else {
-      res[propName] = data[propName];
+      res[propName] = value;
     }
   }
   return res;