diff --git a/app/api/mobile/entities/issue.rb b/app/api/mobile/entities/issue.rb
index 3fa3f311d..7f09906b5 100644
--- a/app/api/mobile/entities/issue.rb
+++ b/app/api/mobile/entities/issue.rb
@@ -1,3 +1,4 @@
+#coding=utf-8
 module Mobile
   module Entities
     class Issue <Grape::Entity
@@ -19,7 +20,7 @@ module Mobile
                 when :issue_priority
                   get_issue_priority_api issue.priority_id
                 when :issue_assigned_to
-                  (get_user(issue.assigned_to_id)).login
+                  issue.assigned_to_id.nil? ? "未指派" : (get_user(issue.assigned_to_id)).login
                 when :issue_status
                   IssueStatus.find(issue.status_id).name
                 when :comment_count