diff --git a/src/infra/heartbeat-runner.response-prefix-template.test.ts b/src/infra/heartbeat-runner.response-prefix-template.test.ts
index f155264ea97..ecde0e248a4 100644
--- a/src/infra/heartbeat-runner.response-prefix-template.test.ts
+++ b/src/infra/heartbeat-runner.response-prefix-template.test.ts
@@ -52,7 +52,7 @@ describe("runHeartbeatOnce responsePrefix templates", () => {
   }
 
   function requireFirstMockCall<T>(mock: { mock: { calls: T[][] } }, label: string): T[] {
-    const call = mock.mock.calls.at(0);
+    const call = mock.mock.calls[0];
     if (!call) {
       throw new Error(`expected ${label} call`);
     }