diff --git a/builtin-rev-parse.c b/builtin-rev-parse.c
index 4377d357c..b3e4386c1 100644
--- a/builtin-rev-parse.c
+++ b/builtin-rev-parse.c
@@ -164,7 +164,7 @@ static int show_file(const char *arg)
 	return 0;
 }
 
-static int try_difference(char *arg)
+static int try_difference(const char *arg)
 {
 	char *dotdot;
 	unsigned char sha1[20];