diff --git a/gandalftest/server.go b/gandalftest/server.go
index 25621b2..566f6c1 100644
--- a/gandalftest/server.go
+++ b/gandalftest/server.go
@@ -225,11 +225,11 @@ func (s *GandalfServer) createUser(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, err.Error(), http.StatusBadRequest)
 		return
 	}
-	s.users = append(s.users, usr.Name)
 	if _, ok := s.keys[usr.Name]; ok {
 		http.Error(w, "user already exists", http.StatusConflict)
 		return
 	}
+	s.users = append(s.users, usr.Name)
 	keys := make([]key, 0, len(usr.Keys))
 	for name, body := range usr.Keys {
 		keys = append(keys, key{Name: name, Body: body})
diff --git a/gandalftest/server_test.go b/gandalftest/server_test.go
index e314b4d..f031469 100644
--- a/gandalftest/server_test.go
+++ b/gandalftest/server_test.go
@@ -174,6 +174,7 @@ func (s *S) TestCreateDuplicateUser(c *check.C) {
 	server.ServeHTTP(recorder, request)
 	c.Assert(recorder.Code, check.Equals, http.StatusConflict)
 	c.Assert(recorder.Body.String(), check.Equals, "user already exists\n")
+	c.Assert(server.Users(), check.DeepEquals, []string{"someuser"})
 }
 
 func (s *S) TestRemoveUser(c *check.C) {