diff -r 12de39a2c580 tools/libxl/libxl_dm.c
--- a/tools/libxl/libxl_dm.c	Thu Feb 10 15:18:51 2011 +0100
+++ b/tools/libxl/libxl_dm.c	Fri Feb 11 11:03:59 2011 +0100
@@ -74,7 +74,7 @@ static char ** libxl_build_device_model_
         } else {
             vncarg = "127.0.0.1:0";
         }
-        if (info->vncpasswd)
+        if (info->vncpasswd && (info->vncpasswd[0] != '\0'))
             vncarg = libxl__sprintf(gc, "%s,password", vncarg);
         flexarray_append(dm_args, "-vnc");
         flexarray_append(dm_args, vncarg);