twitter bootstrap accordion IE 9 troubles with selectbox

twitter bootstrap accordion IE 9 troubles with selectbox

I'm using angularJS with twitter bootstrap. in my html page i'm using
accordion and in the content of the accordion I have a select box. it
works fine with firfox, ie10, chrome ... but in IE9 it cuts off the text
in the select box. it only shows the first letter of the text of the
preselected value. if i click in the select box, i can see the whole text.
can anybody tell me, how to fix this problem? it seems to be a problem
with the accordion, because if i put the selectbox outside of the
accordion, the selectbox also works in IE9.