diff --git a/packages/@aws-cdk/cdk/lib/app.ts b/packages/@aws-cdk/cdk/lib/app.ts
index f3e7c6a90ff8a..3892bd72bdc1d 100644
--- a/packages/@aws-cdk/cdk/lib/app.ts
+++ b/packages/@aws-cdk/cdk/lib/app.ts
@@ -1,7 +1,7 @@
 import cxapi = require('@aws-cdk/cx-api');
 import fs = require('fs');
 import path = require('path');
-import { Stack } from './cloudformation/stack';
+import { isStack, Stack } from './cloudformation/stack';
 import { Construct, MetadataEntry, PATH_SEP, Root } from './core/construct';
 import { resolve } from './core/tokens';
 
@@ -21,8 +21,8 @@ export class App extends Root {
   private get stacks() {
     const out: { [name: string]: Stack } = { };
     for (const child of this.children) {
-      if (!(child instanceof Stack)) {
-        throw new Error(`The child ${child.toString()} of Program must be a Stack`);
+      if (!isStack(child)) {
+        throw new Error(`The child ${child.toString()} of App must be a Stack`);
       }
 
       out[child.id] = child as Stack;
diff --git a/packages/@aws-cdk/cdk/lib/cloudformation/stack.ts b/packages/@aws-cdk/cdk/lib/cloudformation/stack.ts
index 5e97d935a8ed6..04182776660cb 100644
--- a/packages/@aws-cdk/cdk/lib/cloudformation/stack.ts
+++ b/packages/@aws-cdk/cdk/lib/cloudformation/stack.ts
@@ -438,7 +438,7 @@ export abstract class Referenceable extends StackElement {
  *
  * We do attribute detection since we can't reliably use 'instanceof'.
  */
-function isStack(construct: Construct): construct is Stack {
+export function isStack(construct: Construct): construct is Stack {
   return (construct as any).isStack;
 }