diff --git a/backend/src/validatorUtils.ts b/backend/src/validatorUtils.ts
index a200566e..1495d8b5 100644
--- a/backend/src/validatorUtils.ts
+++ b/backend/src/validatorUtils.ts
@@ -57,7 +57,7 @@ function getContextPath(context) {
 // tslint:enable
 
 export class StrictValidationError extends Error {
-  private errors;
+  private readonly errors;
 
   constructor(errors: string[]) {
     errors = Array.from(new Set(errors));