Sorting and focus - Tabindex not take in account with IE7

Hello,
I'am working with the datable. I'have an issue with IE7 : the sortable column can't be focusable with the "Tab" key (it's working fine with IE8 and firefox). I checked the generated source code, and the "tabindex" property is correctly put to "0" on each <th> element of the <thead> but it's not workink...

Do you know how can I fix this issue?

Thanks and regards