diff --git a/lib/server/start.js b/lib/server/start.js
index f59ad0e..fded2be 100644
--- a/lib/server/start.js
+++ b/lib/server/start.js
@@ -200,7 +200,7 @@ async function startup(path, connector, omitWatchers, omitTimers, routine) {
         catch (err) {
             console.error(err);
             const { request } = ctx;
-            const exception = err instanceof types_1.OakException
+            const exception = (0, types_1.isOakException)(err)
                 ? err
                 : new types_1.OakException(serverConfiguration?.internalExceptionMask ||
                     ExceptionMask);
diff --git a/src/server/start.ts b/src/server/start.ts
index ad1ad64..126bcd9 100644
--- a/src/server/start.ts
+++ b/src/server/start.ts
@@ -9,7 +9,7 @@ import KoaBody from 'koa-body';
 
 import { AppLoader, getClusterInfo, ClusterAppLoader } from 'oak-backend-base';
 import { BackendRuntimeContext } from 'oak-frontend-base/lib/context/BackendRuntimeContext';
-import { OakException, Connector, EntityDict, ClusterInfo, OpRecord } from 'oak-domain/lib/types';
+import { OakException, Connector, EntityDict, ClusterInfo, OpRecord, isOakException } from 'oak-domain/lib/types';
 import { EntityDict as BaseEntityDict } from 'oak-domain/lib/base-app-domain';
 import { AsyncRowStore, AsyncContext } from 'oak-domain/lib/store/AsyncRowStore';
 import { SyncContext } from 'oak-domain/lib/store/SyncRowStore';
@@ -245,7 +245,7 @@ export async function startup<ED extends EntityDict & BaseEntityDict, FrontCxt e
             console.error(err);
             const { request } = ctx;
             const exception =
-                err instanceof OakException
+                 isOakException(err)
                     ? err
                     : new OakException<ED>(
                         serverConfiguration?.internalExceptionMask ||