diff --git a/app/services/grade_service.ts b/app/services/grade_service.ts
index 4751016..16191e3 100644
--- a/app/services/grade_service.ts
+++ b/app/services/grade_service.ts
@@ -3,11 +3,12 @@ import { DateTime } from 'luxon'
 
 export class GradeService {
   private calculateAverage(colles: Colle[]) {
-    const total = colles
+    const grades = colles
       .map((colle) => parseFloat(colle.grade?.toString()))
-      .filter(Boolean)
-      .reduce((sum, grade) => sum + grade, 0)
-    return (total / colles.length).toFixed(2)
+      .filter((g) => !isNaN(g) && g !== undefined)
+    if (grades.length === 0) return '0.00'
+    const total = grades.reduce((sum, g) => sum + g, 0)
+    return (total / grades.length).toFixed(2)
   }
 
   private calculateSubjectAverage(colles: Colle[], subject: string) {