diff --git a/app/services/username_checker_service.rb b/app/services/username_checker_service.rb
index 71c6d9a2b79..9584cc63ada 100644
--- a/app/services/username_checker_service.rb
+++ b/app/services/username_checker_service.rb
@@ -11,7 +11,12 @@ class UsernameCheckerService
         check_username_with_hub_server(username, email)
       end
     elsif email and SiteSetting.call_discourse_hub?
-      {suggestion: DiscourseHub.nickname_for_email(email)}
+      username_from_hub = DiscourseHub.nickname_for_email(email)
+      if username_from_hub && User.username_available?(username_from_hub)
+        {suggestion: username_from_hub}
+      else
+        {suggestion: nil}
+      end
     end
   end
 
diff --git a/spec/services/username_checker_service_spec.rb b/spec/services/username_checker_service_spec.rb
index 7f802ec6b01..0d3fdfe2de4 100644
--- a/spec/services/username_checker_service_spec.rb
+++ b/spec/services/username_checker_service_spec.rb
@@ -95,6 +95,14 @@ describe UsernameCheckerService do
           DiscourseHub.stubs(:nickname_for_email).returns('vincent')
           result.should == {suggestion: 'vincent'}
         end
+
+        it 'match found for email, but username is taken' do
+          # This case can happen when you've already signed up on the site,
+          # or enforce_global_nicknames used to be disabled.
+          DiscourseHub.stubs(:nickname_for_email).returns('taken')
+          User.stubs(:username_available?).with('taken').returns(false)
+          result.should == {suggestion: nil}
+        end
       end
 
       context 'username is nil' do