Error page is not working on iis server

Question

IIS server is not redirecting to the error page if the wrong url is not an .asp ending file.

Eg:

If the entered url is

http://domain.com/checking

The error page is not displayed, instead a default page of iis is displayed

If the url is http://domain.com/checking.aspx

and the checking.aspx is not available, then the user gets directed to error page.

Need help with the webconfig of asp.

<configuration>
  <system.web> 
    <compilation debug="true" targetFramework="4.5"/>
    <httpRuntime targetFramework="4.5"/>
    <customErrors mode="On" defaultRedirect="404.aspx">
      <error statusCode="404" redirect="404.aspx" />
      <error statusCode="500" redirect="404.aspx" />
    </customErrors>
  </system.web>
</configuration>