diff --git a/js/molecule-polarity-config.js b/js/molecule-polarity-config.js
index 40c8af84..0f67d175 100644
--- a/js/molecule-polarity-config.js
+++ b/js/molecule-polarity-config.js
@@ -29,6 +29,7 @@ require.config( {
     KITE: '../../kite/js',
     NITROGLYCERIN: '../../nitroglycerin/js',
     PHET_CORE: '../../phet-core/js',
+    PHET_IO: '../../phet-io/js',
     PHETCOMMON: '../../phetcommon/js',
     REPOSITORY: '..',
     SCENERY: '../../scenery/js',