diff --git a/plugins/labhub.py b/plugins/labhub.py
index 1d702b71..ca0b3502 100644
--- a/plugins/labhub.py
+++ b/plugins/labhub.py
@@ -122,6 +122,9 @@ def invite_cmd(self, msg, match):
         """
         invitee = match.group(1)
         inviter = msg.frm.nick
+        if not inviter:
+            yield 'ERROR: The above command cannot be operated without nick.'
+            return
 
         team = 'newcomers' if match.group(2) is None else match.group(2)
         team = team.lower()
@@ -196,6 +199,9 @@ def callback_message(self, msg):
     def create_issue_cmd(self, msg, match):
         """Create issues on GitHub and GitLab repositories."""  # Ignore QuotesBear, LineLengthBear, PyCodeStyleBear
         user = msg.frm.nick
+        if not user:
+            yield 'ERROR: The above command cannot be operated without nick.'
+            return
         repo_name = match.group(1)
         iss_title = match.group(2)
         iss_description = match.group(3) if match.group(3) is not None else ''
@@ -234,6 +240,9 @@ def unassign_cmd(self, msg, match):
         issue_number = match.group(4)
 
         user = msg.frm.nick
+        if not user:
+            yield 'ERROR: The above command cannot be operated without nick.'
+            return
 
         try:
             assert org == self.GH_ORG_NAME or org == self.GL_ORG_NAME
@@ -317,6 +326,9 @@ def assign_cmd(self, msg, match):
         iss_number = match.group(4)
 
         user = msg.frm.nick
+        if not user:
+            yield 'ERROR: The above command cannot be operated without nick.'
+            return
 
         try:
             assert org == self.GH_ORG_NAME or org == self.GL_ORG_NAME
diff --git a/tests/labhub_test.py b/tests/labhub_test.py
index 80492ce4..cac4130e 100644
--- a/tests/labhub_test.py
+++ b/tests/labhub_test.py
@@ -33,6 +33,7 @@ def setUp(self):
             'GH_ORG_NAME': 'coala',
             'GL_ORG_NAME': 'coala',
         }
+        self.bot.sender._nick = 'batman'
         self.labhub = self.load_plugin('LabHub', self.global_mocks, configs)
 
     def test_invite_cmd(self):
@@ -102,6 +103,11 @@ def test_invite_cmd(self):
         testbot.assertCommand('!invite meetto newcomers',
                               'Command "invite" / "invite meetto" not found.')
 
+        self.bot.sender._nick = None
+        testbot.assertCommand(
+            '!invite meet to newcomers',
+            'ERROR: The above command cannot be operated without nick.')
+
         # not a member of org
         mock_team_newcomers.is_member.return_value = False
         mock_team_developers.is_member.return_value = False
@@ -156,7 +162,7 @@ def test_create_issue_cmd(self):
                 textwrap.dedent('''\
                     first line of body
                     second line of body
-                    Opened by @None at [text]()''')
+                    Opened by @batman at [text]()''')
         )
 
         testbot_public.assertCommand(
@@ -171,13 +177,18 @@ def test_create_issue_cmd(self):
                 'another title',
                 textwrap.dedent('''\
                     body
-                    Opened by @None at [text]()''')
+                    Opened by @batman at [text]()''')
         )
 
         testbot_public.assertCommand(
             '!new issue coala title',
             'repository that does not exist')
 
+        self.bot.sender._nick = None
+        testbot_public.assertCommand(
+            '!new issue coala title',
+            'ERROR: The above command cannot be operated without nick.')
+
         # not a member of org
         self.mock_team.is_member.return_value = False
         testbot_public.assertCommand(
@@ -201,7 +212,7 @@ def test_unassign_cmd(self):
         mock_iss = create_autospec(IGitt.GitHub.GitHubIssue)
         self.mock_repo.get_issue.return_value = mock_iss
         mock_iss.assignees = PropertyMock()
-        mock_iss.assignees = (None, )
+        mock_iss.assignees = ('batman', )
         mock_iss.unassign = MagicMock()
         self.mock_team.is_member.return_value = True
         testbot = self
@@ -211,7 +222,7 @@ def test_unassign_cmd(self):
             'you are unassigned now',
             timeout=10000)
         self.mock_repo.get_issue.assert_called_with(999)
-        mock_iss.unassign.assert_called_once_with(None)
+        mock_iss.unassign.assert_called_once_with('batman')
 
         mock_iss.assignees = ('meetmangukiya', )
         testbot.assertCommand(
@@ -226,6 +237,11 @@ def test_unassign_cmd(self):
             '!unassign https://gitlab.com/example/test/issues/999',
             'Repository not owned by our org.')
 
+        self.bot.sender._nick = None
+        testbot.assertCommand(
+            '!unassign https://gitlab.com/example/test/issues/999',
+            'ERROR: The above command cannot be operated without nick.')
+
         # not a member of org
         self.mock_team.is_member.return_value = False
         testbot.assertCommand(
@@ -253,6 +269,13 @@ def test_assign_cmd(self):
         testbot = self
 
         cmd = '!assign https://github.com/{}/{}/issues/{}'
+
+        self.bot.sender._nick = None
+        testbot.assertCommand(
+            cmd.format('coa', 'a', '23'),
+            'ERROR: The above command cannot be operated without nick.')
+        self.bot.sender._nick = 'batman'
+
         # no assignee, not newcomer
         mock_issue.assignees = tuple()
         self.mock_team.is_member.return_value = False
@@ -335,7 +358,7 @@ def test_assign_cmd(self):
                               'already assigned to someone')
 
         # has assignee same as user
-        mock_issue.assignees = (None, )
+        mock_issue.assignees = ('batman', )
         testbot.assertCommand(cmd.format('coala', 'a', '23'),
                               'already assigned to you')