Invalid URL

Hello,

Thanks so much for this awesome plugin! I have a question regarding the URL field. When I submit an form and enter a link, it always comes back as an invalid URL on the first try. I’ve submitted it as ‘http://example.com’ and corrected it to ‘http://example.com/’ (it worked). When I submitted another form with ‘http://example.com/’ it said it was invalid, so I entered ‘http://example.com’ and it worked. What makes a valid url?

Thanks

http://ww.wp.xz.cn/plugins/si-contact-form/