diff --git a/optuna_dashboard/ts/graphUtil.ts b/optuna_dashboard/ts/graphUtil.ts
index c9829a27..7131032f 100644
--- a/optuna_dashboard/ts/graphUtil.ts
+++ b/optuna_dashboard/ts/graphUtil.ts
@@ -45,7 +45,7 @@ const getAxisInfoForCategoricalParams = (
   )
 
   const indices = distribution.choices
-    .map((c) => c?.value.toString() ?? "null")
+    .map((c) => c?.value ?? "null")
     .sort((a, b) =>
       a.toLowerCase() < b.toLowerCase()
         ? -1