diff --git a/lib/Service/AssignmentService.php b/lib/Service/AssignmentService.php
index dd8d8f17a..fb0851426 100644
--- a/lib/Service/AssignmentService.php
+++ b/lib/Service/AssignmentService.php
@@ -62,7 +62,7 @@ public function assignUser(int $cardId, string $userId, int $type = Assignment::
 
 		$card = $this->cardMapper->find($cardId);
 		$boardId = $this->cardMapper->findBoardId($cardId);
-		$boardUsers = array_keys($this->permissionService->findUsers($boardId, true));
+		$boardUsers = array_map(fn (string|int $userId) => (string)$userId, array_keys($this->permissionService->findUsers($boardId, true)));
 		$acls = $this->aclMapper->findAll($boardId);
 		$groups = array_filter($acls, function (Acl $acl) use ($userId) {
 			return $acl->getType() === Acl::PERMISSION_TYPE_GROUP && $acl->getParticipant() === $userId;