diff --git a/src/activitypub/helpers.js b/src/activitypub/helpers.js
index 04118b8f63..7153deb6c9 100644
--- a/src/activitypub/helpers.js
+++ b/src/activitypub/helpers.js
@@ -111,7 +111,7 @@ Helpers.resolveLocalId = async (input) => {
 		}
 
 		return { type: null, id: null };
-	} else if (input.indexOf('@') !== -1) { // Webfinger
+	} else if (String(input).indexOf('@') !== -1) { // Webfinger
 		const [slug] = input.replace(/^acct:/, '').split('@');
 		const uid = await user.getUidByUserslug(slug);
 		return { type: 'user', id: uid };