diff --git a/dot_tmux.conf b/dot_tmux.conf
index 06626a1..b5b3e55 100644
--- a/dot_tmux.conf
+++ b/dot_tmux.conf
@@ -36,7 +36,7 @@ if-shell '[ -x /usr/bin/fish ]' {
     set -g default-shell /usr/bin/fish
 }
 # use custom shell if configured with ENV
-if-shell '[ -x $TMUX_SHELL ]' {
+if-shell -F $TMUX_SHELL {
     set -g default-shell $TMUX_SHELL
 }