diff --git a/lib/mysql.js b/lib/mysql.js
index 3c4831e..5f3cafc 100644
--- a/lib/mysql.js
+++ b/lib/mysql.js
@@ -269,7 +269,7 @@ function dateToMysql(val) {
  */
 MySQL.prototype.toDatabase = function (prop, val) {
   if (val === null) return 'NULL';
-  if (val === undefined) return;
+  if (val === undefined) return 'NULL';
   if (val.constructor.name === 'Object') {
     var operator = Object.keys(val)[0]
     val = val[operator];