diff --git a/packages/testing/src/mocking-activity-environment.ts b/packages/testing/src/mocking-activity-environment.ts
index 943f5f4b7..b85db15bb 100644
--- a/packages/testing/src/mocking-activity-environment.ts
+++ b/packages/testing/src/mocking-activity-environment.ts
@@ -87,7 +87,7 @@ export const defaultActivityInfo: activity.Info = {
   heartbeatDetails: undefined,
   activityNamespace: 'default',
   workflowNamespace: 'default',
-  workflowExecution: { workflowId: 'test', runId: 'dead-beef' },
+  workflowExecution: { workflowId: 'test', runId: '00000000-0000-0000-0000-000000000000' },
   scheduledTimestampMs: 1,
   startToCloseTimeoutMs: 1000,
   scheduleToCloseTimeoutMs: 1000,