diff --git a/functions.sh b/functions.sh
index b6a8734..fb6e49f 100755
--- a/functions.sh
+++ b/functions.sh
@@ -76,7 +76,7 @@ function _pkcs11module_hook() {
 
     loginfo "PKCS11_MODULE: $PKCS11_MODULE"
 
-    if [ -n "${TMUX}" ]; then
+    if [ -n "${TMUX}" -a -n "${PKCS11_MODULE}" ]; then
         logdebug "set PKCS11_MODULE also in tmux environment"
         tmux set-environment PKCS11_MODULE ${PKCS11_MODULE}
         tmux set-environment -g PKCS11_MODULE ${PKCS11_MODULE}