diff --git a/api/src/engine/models/result/common/chart-axis.model.ts b/api/src/engine/models/result/common/chart-axis.model.ts
index f5045887954c5399f107718ae539bfcc9ac7d739..16fc1f68a10e379b0bf4e45225c7912710d1d34d 100644
--- a/api/src/engine/models/result/common/chart-axis.model.ts
+++ b/api/src/engine/models/result/common/chart-axis.model.ts
@@ -6,5 +6,5 @@ export class ChartAxis {
   label?: string;
 
   @Field(() => [String], { nullable: true, defaultValue: [] })
-  categories: string[];
+  categories?: string[];
 }