diff --git a/optuna_dashboard/ts/apiClient.ts b/optuna_dashboard/ts/apiClient.ts
index 12feaf4a..b03f5691 100644
--- a/optuna_dashboard/ts/apiClient.ts
+++ b/optuna_dashboard/ts/apiClient.ts
@@ -30,7 +30,7 @@ interface TrialResponse {
   system_attrs: Attribute[]
   note: Note
   artifacts: Artifact[]
-  constraints: number[]
+  constraints?: number[]
 }
 
 const convertTrialResponse = (res: TrialResponse): Trial => {
diff --git a/optuna_dashboard/ts/types/index.d.ts b/optuna_dashboard/ts/types/index.d.ts
index 040a1fc3..c49339e0 100644
--- a/optuna_dashboard/ts/types/index.d.ts
+++ b/optuna_dashboard/ts/types/index.d.ts
@@ -112,7 +112,7 @@ type Trial = {
   }[]
   user_attrs: Attribute[]
   system_attrs: Attribute[]
-  constraints: number[]
+  constraints?: number[]
   note: Note
   artifacts: Artifact[]
 }