diff --git a/packages/backend-tasks/src/tasks/PluginTaskSchedulerImpl.ts b/packages/backend-tasks/src/tasks/PluginTaskSchedulerImpl.ts
index 44bb2ae7a6..646107776c 100644
--- a/packages/backend-tasks/src/tasks/PluginTaskSchedulerImpl.ts
+++ b/packages/backend-tasks/src/tasks/PluginTaskSchedulerImpl.ts
@@ -56,7 +56,7 @@ export class PluginTaskSchedulerImpl implements PluginTaskScheduler {
       return frequency.cron;
     }
 
-    if (frequency instanceof Duration) {
+    if (Duration.isDuration(frequency)) {
       return frequency.toISO();
     }