Not Require Key Word Input When Searching
 

Does anyone know how to make it so that users are not required to add a key word when making a search? Right now I get this error:

I have extra threadfields to search, but do not want users to have to enter in a keyword. See here for an example, I want users to be able to check a box and execute a search:

http://www.onereef.com/forums/search.php

I'd really appreciate it if someone could help answer this. I know it's a simple line change, I just do not know where. Someone was helpful a few months ago and answered the question, yet he closed his entire forum down and now the record of it is gone. This is the last element to allow me to launch my forum!