Sidebar crashing into footer if content-area is shorter in height

Got a problem with a theme I tweaked from Twenty Thirteen. Problem is on some pages like this one, the sidebar is crashing into the footer if the main content area is shorter in height than the sidebar. Which happens a couple of times. I have a clear: both; in the footer div so I don’t understand why this is occurring. How can I clear this up so there’s no crashing?