diff --git a/builtin-merge.c b/builtin-merge.c
index e5cf795307..2871419143 100644
--- a/builtin-merge.c
+++ b/builtin-merge.c
@@ -70,7 +70,7 @@ static int option_parse_message(const struct option *opt,
 	if (unset)
 		strbuf_setlen(buf, 0);
 	else if (arg) {
-		strbuf_addf(buf, "%s\n\n", arg);
+		strbuf_addf(buf, "%s%s", buf->len ? "\n\n" : "", arg);
 		have_message = 1;
 	} else
 		return error("switch `m' requires a value");
@@ -927,11 +927,13 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		 * codepath so we discard the error in this
 		 * loop.
 		 */
-		for (i = 0; i < argc; i++)
-			merge_name(argv[i], &msg);
-		fmt_merge_msg(option_log, &msg, &merge_msg);
-		if (merge_msg.len)
-			strbuf_setlen(&merge_msg, merge_msg.len-1);
+		if (!have_message) {
+			for (i = 0; i < argc; i++)
+				merge_name(argv[i], &msg);
+			fmt_merge_msg(option_log, &msg, &merge_msg);
+			if (merge_msg.len)
+				strbuf_setlen(&merge_msg, merge_msg.len-1);
+		}
 	}
 
 	if (head_invalid || !argc)
diff --git a/t/t7604-merge-custom-message.sh b/t/t7604-merge-custom-message.sh
index de977c5e2f..269cfdf267 100755
--- a/t/t7604-merge-custom-message.sh
+++ b/t/t7604-merge-custom-message.sh
@@ -22,15 +22,12 @@ test_expect_success 'setup' '
 	git tag c2
 '
 
-cat >expected <<\EOF
-custom message
 
-Merge commit 'c2'
-EOF
 test_expect_success 'merge c2 with a custom message' '
 	git reset --hard c1 &&
+	echo >expected "custom message" &&
 	git merge -m "custom message" c2 &&
-	git cat-file commit HEAD | sed -e "1,/^$/d" > actual &&
+	git cat-file commit HEAD | sed -e "1,/^$/d" >actual &&
 	test_cmp expected actual
 '