diff --git a/object-file.c b/object-file.c
index 2bc36ab3ee..6ff6cf7549 100644
--- a/object-file.c
+++ b/object-file.c
@@ -1296,7 +1296,11 @@ int index_path(struct index_state *istate, struct object_id *oid,
 		strbuf_release(&sb);
 		break;
 	case S_IFDIR:
-		return repo_resolve_gitlink_ref(istate->repo, path, "HEAD", oid);
+		if (repo_resolve_gitlink_ref(istate->repo, path, "HEAD", oid))
+			return -1;
+		if (&hash_algos[oid->algo] != istate->repo->hash_algo)
+			return error(_("cannot add a submodule of a different hash algorithm"));
+		break;
 	default:
 		return error(_("%s: unsupported file type"), path);
 	}
diff --git a/t/t3700-add.sh b/t/t3700-add.sh
index df580a5806..9a2c8dbcc2 100755
--- a/t/t3700-add.sh
+++ b/t/t3700-add.sh
@@ -541,6 +541,31 @@ test_expect_success 'all statuses changed in folder if . is given' '
 	)
 '
 
+test_expect_success 'cannot add a submodule of a different algorithm' '
+	git init --object-format=sha256 sha256 &&
+	(
+		cd sha256 &&
+		test_commit abc &&
+		git init --object-format=sha1 submodule &&
+		test_commit -C submodule def &&
+		test_must_fail git add submodule 2>err &&
+		test_grep "cannot add a submodule of a different hash algorithm" err &&
+		git ls-files --stage >entries &&
+		test_grep ! ^160000 entries
+	) &&
+	git init --object-format=sha1 sha1 &&
+	(
+		cd sha1 &&
+		test_commit abc &&
+		git init --object-format=sha256 submodule &&
+		test_commit -C submodule def &&
+		test_must_fail git add submodule 2>err &&
+		test_grep "cannot add a submodule of a different hash algorithm" err &&
+		git ls-files --stage >entries &&
+		test_grep ! ^160000 entries
+	)
+'
+
 test_expect_success CASE_INSENSITIVE_FS 'path is case-insensitive' '
 	path="$(pwd)/BLUB" &&
 	touch "$path" &&
diff --git a/t/t7400-submodule-basic.sh b/t/t7400-submodule-basic.sh
index fd3e7e355e..e6b551daad 100755
--- a/t/t7400-submodule-basic.sh
+++ b/t/t7400-submodule-basic.sh
@@ -407,6 +407,31 @@ test_expect_success 'submodule add in subdirectory with relative path should fai
 	test_grep toplevel output.err
 '
 
+test_expect_success 'submodule add of a different algorithm fails' '
+	git init --object-format=sha256 sha256 &&
+	(
+		cd sha256 &&
+		test_commit abc &&
+		git init --object-format=sha1 submodule &&
+		test_commit -C submodule def &&
+		test_must_fail git submodule add "$submodurl" submodule 2>err &&
+		test_grep "cannot add a submodule of a different hash algorithm" err &&
+		git ls-files --stage >entries &&
+		test_grep ! ^160000 entries
+	) &&
+	git init --object-format=sha1 sha1 &&
+	(
+		cd sha1 &&
+		test_commit abc &&
+		git init --object-format=sha256 submodule &&
+		test_commit -C submodule def &&
+		test_must_fail git submodule add "$submodurl" submodule 2>err &&
+		test_grep "cannot add a submodule of a different hash algorithm" err &&
+		git ls-files --stage >entries &&
+		test_grep ! ^160000 entries
+	)
+'
+
 test_expect_success 'setup - add an example entry to .gitmodules' '
 	git config --file=.gitmodules submodule.example.url git://example.com/init.git
 '