Null Search Results

I have set up the Extended Search app which is working great. I’ve set up a search for “Select Post Types” to a Custom Post Type (Eg, Assets). When I do a search for an actual post title I get the results as expected, however if a carry out a null search, the results show posts from all different post types not limiting to Assets. I am happy for a null search to list all Assets posts, just not other post types.