index.php shows error when setting enable_query_strings to True

in my config.php I have set the enable_query_strings = TRUE

as it will be much easier to display images.

so my url is something like:

localhost/ci/index.php?c=welcome

If I call localhost/ci/index.php?c=welcome or localhost/ci/index.php?c=welcome&m=index it works.

However if I just call:

localhost/ci/index.php or localhost/ci/

I'll have the error message saying

Unable to determine what should be displayed. A default route has not been specified in the routing file.

in my routes.php

$route['default_controller'] = 'welcome';

Please advise