diff --git a/conf/virtualenv_activate b/conf/virtualenv_activate
index 7495379..a3b5883 100644
--- a/conf/virtualenv_activate
+++ b/conf/virtualenv_activate
@@ -28,7 +28,7 @@ deactivate () {
     fi
 
     unset VIRTUAL_ENV
-    if [ ! "$1" = "nondestructive" ] ; then
+    if [ ! "${1:-}" = "nondestructive" ] ; then
     # Self destruct!
         unset -f deactivate
     fi