diff --git a/js/sitl.js b/js/sitl.js
index 2d0c2e2a1..338a3eea4 100644
--- a/js/sitl.js
+++ b/js/sitl.js
@@ -202,7 +202,9 @@ var SITLProcess = {
             }
         }
 
-        callback( sitlExePath + " " + args.join(" ") + "\n");
+	if (callback) {
+            callback( sitlExePath + " " + args.join(" ") + "\n");
+	}
         this.spawn(sitlExePath, args, callback);
     },