diff --git a/lib/connection-pool.js b/lib/connection-pool.js
index c2d96f1..fb7b133 100644
--- a/lib/connection-pool.js
+++ b/lib/connection-pool.js
@@ -254,7 +254,8 @@ ConnectionPool.prototype.release = function(connection) {
 ConnectionPool.prototype.drain = function (callback) {
     this.log('draining pool');
     if (this.drained) {//pool has been drained
-        callback();
+        if (callback)
+            callback();
         return;
     }