diff --git a/fish_right_prompt.fish b/fish_right_prompt.fish
index c0afd97..fa4de43 100644
--- a/fish_right_prompt.fish
+++ b/fish_right_prompt.fish
@@ -3,7 +3,7 @@
 
 function __bobthefish_cmd_duration -S -d 'Show command duration'
   [ "$theme_display_cmd_duration" = "no" ]; and return
-  [ "$CMD_DURATION" -lt 100 ]; and return
+  [ -z "$CMD_DURATION" -o "$CMD_DURATION" -lt 100 ]; and return
 
   if [ "$CMD_DURATION" -lt 5000 ]
     echo -ns $CMD_DURATION 'ms'