TwentyThirteen overlaps footer?

Hi,

I have a child TwentyThirteen theme, and if the sidebar content is longer than the main content then it overlaps into the footer. This has been raised elsewhere but as yet found no answers. My site still on local host but when trying to find an answer I found a site exhibiting the same problem - http://amsyong.com/daftar-isi.

HELP .... please

Thanks
Mark