diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 6e73c85cde..753bd53a70 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -90,7 +90,8 @@ static struct odb_source_files *handle_object_dir_option(struct repository *repo
 {
 	struct odb_source *source = odb_find_source(repo->objects, opts.object_dir);
 	if (!source)
-		source = odb_add_to_alternates_memory(repo->objects, opts.object_dir);
+		die(_("object directory is not an alternate of the current repository: '%s'"),
+		    opts.object_dir);
 	return odb_source_files_downcast(source);
 }
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 68143cb5b7..00e90f163f 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -698,10 +698,9 @@ test_expect_success 'force some 64-bit offsets with pack-objects' '
 	corrupt_data $idx64 $(test_oid idxoff) "\02" &&
 	# objects64 is not a real repository, but can serve as an alternate
 	# anyway so we can write a MIDX into it
-	git init repo &&
-	test_when_finished "rm -fr repo" &&
+	git init repo64 &&
 	(
-		cd repo &&
+		cd repo64 &&
 		( cd ../objects64 && pwd ) >.git/objects/info/alternates &&
 		midx64=$(git multi-pack-index --object-dir=../objects64 write)
 	) &&
@@ -709,7 +708,7 @@ test_expect_success 'force some 64-bit offsets with pack-objects' '
 '
 
 test_expect_success 'verify multi-pack-index with 64-bit offsets' '
-	git multi-pack-index verify --object-dir=objects64
+	git -C repo64 multi-pack-index verify --object-dir=../objects64
 '
 
 NUM_OBJECTS=63
@@ -721,7 +720,7 @@ MIDX_BYTE_LARGE_OFFSET=$(($MIDX_OFFSET_LARGE_OFFSETS + 3))
 
 test_expect_success 'verify incorrect 64-bit offset' '
 	corrupt_midx_and_verify $MIDX_BYTE_LARGE_OFFSET "\07" objects64 \
-		"incorrect object offset"
+		"incorrect object offset" "git -C repo64 multi-pack-index verify --object-dir=../objects64"
 '
 
 test_expect_success 'setup expire tests' '