diff --git a/src/misc/is-objectid.ts b/src/misc/is-objectid.ts
index 8c1aabd56..c3148a778 100644
--- a/src/misc/is-objectid.ts
+++ b/src/misc/is-objectid.ts
@@ -1,3 +1,5 @@
-export default function(x: any): boolean {
+import { ObjectID } from 'mongodb';
+
+export default function(x: any): x is ObjectID {
 	return x.hasOwnProperty('toHexString') || x.hasOwnProperty('_bsontype');
 }