diff --git a/README.md b/README.md
index 8eed10d..baeb90d 100644
--- a/README.md
+++ b/README.md
@@ -8,4 +8,8 @@ See it in action:
 1. add your fallback pinentry to `pinentry-tmux.sh`
 2. put the shell script in your path, eg. `~/.local/bin/pinentry-tmux` (without `.sh`)
 3. configure gpg to use `pinentry-tmux` for pinentry
+4. for the script to detect that it’s running inside of TMUX, add this to your TMUX config:
+   ```sh
+   PINENTRY_USER_DATA="tmux"
+   ```