diff --git a/force_bdss/tests/probe_classes/mco.py b/force_bdss/tests/probe_classes/mco.py
index 1c84b8ee9c350f6df9652fe8136acce8f4cbcb29..eb23e9b0bbbcb48ff12eda280555617041f796db 100644
--- a/force_bdss/tests/probe_classes/mco.py
+++ b/force_bdss/tests/probe_classes/mco.py
@@ -101,7 +101,7 @@ class ProbeMCOFactory(BaseMCOFactory):
             self,
             nb_output_data_values=self.nb_output_data_values)
 
-    def create_model(self, model_data):
+    def create_model(self, model_data=None):
         if self.raises_on_create_model:
             raise Exception("ProbeMCOFactory.create_model")