diff --git a/themes/agnoster.zsh-theme b/themes/agnoster.zsh-theme
index 2db56526..2836de4b 100644
--- a/themes/agnoster.zsh-theme
+++ b/themes/agnoster.zsh-theme
@@ -137,7 +137,7 @@ prompt_dir() {
 # Virtualenv: current working virtualenv
 prompt_virtualenv() {
   local virtualenv_path="$VIRTUAL_ENV"
-  if [[ -n $virtualenv_path && -z $VIRTUAL_ENV_DISABLE_PROMPT ]]; then
+  if [[ -n $virtualenv_path && -n $VIRTUAL_ENV_DISABLE_PROMPT ]]; then
     prompt_segment blue black "(`basename $virtualenv_path`)"
   fi
 }