diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index d4b77fd128..3814731f54 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -1617,7 +1617,7 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 			die(_("packfile name '%s' does not end with '.pack'"),
 			    pack_name);
 		strbuf_add(&keep_name_buf, pack_name, len);
-		strbuf_addstr(&keep_name_buf, ".idx");
+		strbuf_addstr(&keep_name_buf, ".keep");
 		keep_name = keep_name_buf.buf;
 	}
 	if (verify) {
diff --git a/t/t5300-pack-object.sh b/t/t5300-pack-object.sh
index 20c1961515..0e3cadfd57 100755
--- a/t/t5300-pack-object.sh
+++ b/t/t5300-pack-object.sh
@@ -284,6 +284,12 @@ test_expect_success \
      git index-pack test-3.pack &&
      cmp test-3.idx test-3-${packname_3}.idx &&
 
+     cat test-1-${packname_1}.pack >test-4.pack &&
+     rm -f test-4.keep &&
+     git index-pack --keep=why test-4.pack &&
+     cmp test-1-${packname_1}.idx test-4.idx &&
+     test -f test-4.keep &&
+
      :'
 
 test_expect_success 'unpacking with --strict' '