diff --git a/packages/taler-util/src/wallet-types.ts b/packages/taler-util/src/wallet-types.ts
index af775a54e..fc383acb0 100644
--- a/packages/taler-util/src/wallet-types.ts
+++ b/packages/taler-util/src/wallet-types.ts
@@ -597,7 +597,7 @@ export interface WalletDiagnostics {
 
 export interface TalerErrorDetail {
   code: TalerErrorCode;
-  when: AbsoluteTime;
+  when?: AbsoluteTime;
   hint?: string;
   [x: string]: unknown;
 }