diff --git a/optimas/generators/ax/developer/multitask.py b/optimas/generators/ax/developer/multitask.py
index 090f7ddc..0e04fdfd 100644
--- a/optimas/generators/ax/developer/multitask.py
+++ b/optimas/generators/ax/developer/multitask.py
@@ -369,7 +369,7 @@ def _create_experiment(self) -> MultiTypeExperiment:
 
         # Register metric in order to be able to save experiment to json file.
         _, encoder_registry, decoder_registry = register_metrics(
-            {AxMetric, None}
+            {AxMetric: None}
         )
         self._encoder_registry = encoder_registry
         self._decoder_registry = decoder_registry