diff --git a/src/api/server_server.rs b/src/api/server_server.rs
index c5cbf5fa..26e84d08 100644
--- a/src/api/server_server.rs
+++ b/src/api/server_server.rs
@@ -63,7 +63,7 @@ use ruma::{
     uint, user_id, CanonicalJsonObject, CanonicalJsonValue, EventId,
     MilliSecondsSinceUnixEpoch, OwnedEventId, OwnedRoomId, OwnedServerName,
     OwnedServerSigningKeyId, OwnedSigningKeyId, OwnedUserId, RoomId,
-    ServerName,
+    ServerName, UserId,
 };
 use serde_json::value::{to_raw_value, RawValue as RawJsonValue};
 use tokio::sync::RwLock;
@@ -1797,7 +1797,7 @@ pub(crate) async fn create_invite_route(
         Error::BadRequest(ErrorKind::InvalidParam, "sender is not a user id.")
     })?;
 
-    let invited_user: Box<_> = serde_json::from_value(
+    let invited_user: Box<UserId> = serde_json::from_value(
         signed_event
             .get("state_key")
             .ok_or(Error::BadRequest(