diff --git a/builtin-merge.c b/builtin-merge.c
index 06b9970306..e97c79e60c 100644
--- a/builtin-merge.c
+++ b/builtin-merge.c
@@ -66,10 +66,11 @@ static int option_parse_message(const struct option *opt,
 
 	if (unset)
 		strbuf_setlen(buf, 0);
-	else {
+	else if (arg) {
 		strbuf_addf(buf, "%s\n\n", arg);
 		have_message = 1;
-	}
+	} else
+		return error("switch `m' requires a value");
 	return 0;
 }
 
diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index 26cf8dc7c1..5eeb6c2b27 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -221,7 +221,7 @@ test_expect_success 'setup' '
 
 test_debug 'gitk --all'
 
-test_expect_failure 'test option parsing' '
+test_expect_success 'test option parsing' '
 	test_must_fail git merge -$ c1 &&
 	test_must_fail git merge --no-such c1 &&
 	test_must_fail git merge -s foobar c1 &&