diff --git a/src/dialogs.py b/src/dialogs.py
index 324f13dc7..68f953a0a 100644
--- a/src/dialogs.py
+++ b/src/dialogs.py
@@ -2482,7 +2482,7 @@ class JoinGroupchatWindow:
             return
 
         if gajim.contacts.get_contact(self.account, room_jid) and \
-        gajim.contacts.get_contact(self.account, room_jid).is_groupchat():
+        not gajim.contacts.get_contact(self.account, room_jid).is_groupchat():
             ErrorDialog(_('This is not a group chat'),
                 _('%s is not the name of a group chat.') % room_jid)
             return
diff --git a/src/gui_interface.py b/src/gui_interface.py
index ef551d49a..92bc7a8be 100644
--- a/src/gui_interface.py
+++ b/src/gui_interface.py
@@ -1903,7 +1903,7 @@ class Interface:
         """
 
         if gajim.contacts.get_contact(account, room_jid) and \
-        gajim.contacts.get_contact(account, room_jid).is_groupchat():
+        not gajim.contacts.get_contact(account, room_jid).is_groupchat():
             dialogs.ErrorDialog(_('This is not a group chat'),
                 _('%s is not the name of a group chat.') % room_jid)
             return