Blank page

i am using

<filteringRule name="BlockSqlCommands" scanQueryString="true">

 <denyStrings>
            <add string="char" />
            <add string="alter" />

when ever there is url where  query has

http://localhost/city=%

or

http://localhost//Daihatsu/Charade/

i get blank page i rather to use my 404 error page

please help thank you