diff --git a/themes/brethil.zsh-theme b/themes/brethil.zsh-theme
index 2592b75..cba3ebd 100644
--- a/themes/brethil.zsh-theme
+++ b/themes/brethil.zsh-theme
@@ -41,7 +41,7 @@ function job_prompt() {
 
 function shlvl_prompt() {
   local SHLVL_PROMPT
-  if [[ -n "$BYOBU_PREFIX" ]]; then
+  if [[ -n "$TMUX" ]]; then
     SHLVL_PROMPT="%(3L.%K{161}%F{255}%B $((SHLVL-1)) %f%b%k.)" # Shows SHLVL on a magenta background if SHLVL > 1
   else
     SHLVL_PROMPT="%(2L.%K{161}%F{255}%B %L %f%b%k.)" # Shows SHLVL on a magenta background if SHLVL > 1