diff --git a/services/IssueService.js b/services/IssueService.js
index 85c2ae1..0cc1f92 100755
--- a/services/IssueService.js
+++ b/services/IssueService.js
@@ -658,6 +658,11 @@ async function handleIssueUnAssignment(event, issue) {
 
     if (dbIssue.assignee) {
       const assigneeUserId = gitHelper.getUserIdByLogin(event, dbIssue.assignee);
+      if (!assigneeUserId) {
+        // The assignement of this user was failed and broken.
+        // We don't need to handle the unassignment.
+        return;
+      }
       logger.debug(`Looking up TC handle of git user: ${assigneeUserId}`);
       const userMapping = await userService.getTCUserName(event.provider, assigneeUserId);
 
diff --git a/services/UserService.js b/services/UserService.js
index 15bdaf2..19e634a 100755
--- a/services/UserService.js
+++ b/services/UserService.js
@@ -39,6 +39,9 @@ async function getTCUserName(provider, gitUser) {
       criteria.gitlabUsername = gitUser;
     }
   }
+  if (_.isEmpty(criteria)) {
+    throw new Error('Can\'t find the TCUserName. Invalid gitUser.');
+  }
   return await dbHelper.scanOne(models.UserMapping, criteria);
 }