diff --git a/src/finalisers/cjs.js b/src/finalisers/cjs.js
index c71f59d..07adf72 100644
--- a/src/finalisers/cjs.js
+++ b/src/finalisers/cjs.js
@@ -6,7 +6,7 @@ export default function cjs ( bundle, magicString, { exportMode }, options ) {
 	const hasDefaultImport = bundle.externalModules.some( mod => mod.declarations.default);
 
 	if (hasDefaultImport) {
-		intro += `function _interopDefault (ex) { return 'default' in ex ? ex['default'] : ex; }\n\n`;
+		intro += `function _interopDefault (ex) { return (ex && (typeof ex === 'object') && 'default' in ex) ? ex['default'] : ex; }\n\n`;
 	}
 
 	// TODO handle empty imports, once they're supported
diff --git a/test/form/external-imports-custom-names/_expected/cjs.js b/test/form/external-imports-custom-names/_expected/cjs.js
index 8f6506e..9600acd 100644
--- a/test/form/external-imports-custom-names/_expected/cjs.js
+++ b/test/form/external-imports-custom-names/_expected/cjs.js
@@ -1,6 +1,6 @@
 'use strict';
 
-function _interopDefault (ex) { return 'default' in ex ? ex['default'] : ex; }
+function _interopDefault (ex) { return (ex && (typeof ex === 'object') && 'default' in ex) ? ex['default'] : ex; }
 
 var $ = _interopDefault(require('jquery'));
 
diff --git a/test/form/external-imports/_expected/cjs.js b/test/form/external-imports/_expected/cjs.js
index a3ee9b0..94dbdae 100644
--- a/test/form/external-imports/_expected/cjs.js
+++ b/test/form/external-imports/_expected/cjs.js
@@ -1,6 +1,6 @@
 'use strict';
 
-function _interopDefault (ex) { return 'default' in ex ? ex['default'] : ex; }
+function _interopDefault (ex) { return (ex && (typeof ex === 'object') && 'default' in ex) ? ex['default'] : ex; }
 
 var factory = _interopDefault(require('factory'));
 var baz = require('baz');