diff --git a/Makefile b/Makefile
index 6552a8f4edf7a5c877841b3c82a557c8d8d53791..67f37b849e9fd7e33234ded67777fcf903ecd465 100644
--- a/Makefile
+++ b/Makefile
@@ -666,13 +666,13 @@ TEST_BUILTINS_OBJS += test-hashmap.o
 TEST_BUILTINS_OBJS += test-index-version.o
 TEST_BUILTINS_OBJS += test-lazy-init-name-hash.o
 TEST_BUILTINS_OBJS += test-match-trees.o
+TEST_BUILTINS_OBJS += test-mergesort.o
 TEST_BUILTINS_OBJS += test-sha1.o
 
 TEST_PROGRAMS_NEED_X += test-dump-fsmonitor
 TEST_PROGRAMS_NEED_X += test-dump-untracked-cache
 TEST_PROGRAMS_NEED_X += test-fake-ssh
 TEST_PROGRAMS_NEED_X += test-line-buffer
-TEST_PROGRAMS_NEED_X += test-mergesort
 TEST_PROGRAMS_NEED_X += test-mktemp
 TEST_PROGRAMS_NEED_X += test-online-cpus
 TEST_PROGRAMS_NEED_X += test-parse-options
diff --git a/t/helper/test-mergesort.c b/t/helper/test-mergesort.c
index 335cf6b6264cdaf9563736fbcfa40e7a3006a432..c5cffaa4b73ff52b2f166231ceb4a77b24ee8cef 100644
--- a/t/helper/test-mergesort.c
+++ b/t/helper/test-mergesort.c
@@ -1,3 +1,4 @@
+#include "test-tool.h"
 #include "cache.h"
 #include "mergesort.h"
 
@@ -22,7 +23,7 @@ static int compare_strings(const void *a, const void *b)
 	return strcmp(x->text, y->text);
 }
 
-int cmd_main(int argc, const char **argv)
+int cmd__mergesort(int argc, const char **argv)
 {
 	struct line *line, *p = NULL, *lines = NULL;
 	struct strbuf sb = STRBUF_INIT;
diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c
index 16c78833a4e7598b3406fa070d18ece908d87856..5d470158b1466ea21c803212d18843bf521cc2e6 100644
--- a/t/helper/test-tool.c
+++ b/t/helper/test-tool.c
@@ -21,6 +21,7 @@ static struct test_cmd cmds[] = {
 	{ "index-version", cmd__index_version },
 	{ "lazy-init-name-hash", cmd__lazy_init_name_hash },
 	{ "match-trees", cmd__match_trees },
+	{ "mergesort", cmd__mergesort },
 	{ "sha1", cmd__sha1 },
 };
 
diff --git a/t/helper/test-tool.h b/t/helper/test-tool.h
index d2f009cb3e409d5541b6f93a6b3604b69a2022f1..6c9437ff5eaf28578cf0254e4e0dfbf295fb204d 100644
--- a/t/helper/test-tool.h
+++ b/t/helper/test-tool.h
@@ -15,6 +15,7 @@ int cmd__hashmap(int argc, const char **argv);
 int cmd__index_version(int argc, const char **argv);
 int cmd__lazy_init_name_hash(int argc, const char **argv);
 int cmd__match_trees(int argc, const char **argv);
+int cmd__mergesort(int argc, const char **argv);
 int cmd__sha1(int argc, const char **argv);
 
 #endif