diff --git a/pkg/suggestion/v1beta1/optuna/service.py b/pkg/suggestion/v1beta1/optuna/service.py
index 7ebd8b154e0..37952c678b9 100644
--- a/pkg/suggestion/v1beta1/optuna/service.py
+++ b/pkg/suggestion/v1beta1/optuna/service.py
@@ -141,7 +141,7 @@ def _tell(self, trials):
 
     def _get_assignments_key(self, assignments):
         assignments = sorted(assignments, key=lambda a: a.name)
-        assignments_str = [str(a) for a in assignments]
+        assignments_str = [f"{a.name}:{a.value}" for a in assignments]
         return ",".join(assignments_str)
 
     def _get_optuna_search_space(self):