diff --git a/git-mergetool--lib.sh b/git-mergetool--lib.sh
index 5b6278572a..51dd0d67ba 100644
--- a/git-mergetool--lib.sh
+++ b/git-mergetool--lib.sh
@@ -1,3 +1,4 @@
+#!/bin/sh
 # git-mergetool--lib is a library for common merge tool functions
 diff_mode() {
 	test "$TOOL_MODE" = diff