Make unwrapExports only pick out default when it exists

src/helpers.js

@@ -8,7 +8,7 @@ export function commonjsRequire () {
 }
 
 export function unwrapExports (x) {
-	return x && x.__esModule ? x['default'] : x;
+	return x && x.__esModule && Object.prototype.hasOwnProperty.call(x, 'default') ? x['default'] : x;
 }
 
 export function createCommonjsModule(fn, module) {

test/samples/use-own-output/from-rollup.js

@@ -0,0 +1,2 @@
+Object.defineProperty(exports, '__esModule', { value: true });
+exports.x = 10

test/samples/use-own-output/main.js

@@ -0,0 +1,2 @@
+import * as b from "./from-rollup";
+window.b = b;

test/test.js

@@ -420,5 +420,17 @@ describe( 'rollup-plugin-commonjs', () => {
 			assert.notEqual( code.indexOf( 'var validVar' ), -1 );
 			assert.notEqual( code.indexOf( 'var nonIndex' ), -1 );
 		});
+
+		it( 'does not misassign default when consuming rollup output', async () => {
+			// Issue #224
+			const bundle = await rollup({
+				entry: 'samples/use-own-output/main.js',
+				plugins: [ commonjs() ],
+			});
+
+			const window = {};
+			await executeBundle( bundle, { context: { window } } );
+			assert.notEqual( window.b.default, undefined );
+		});
 	});
 });