403 Forbidden when GET parameter name and value are integers

does anyone know what is causing server to issue 403 when GET parameter name and value are integers, for example:

http://example.com?1=1 bad http://example.com?a=b&1=2&开发者_JAVA技巧;c=d bad http://example.com?a=b&c=d good

Apache mod_fcgid/2.3.6 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 Server at www.example.com Port 80

Thanks.