Dropdown bugged in IE 6 and 7

The hover dropdown seems to be nonfunctional in IE 6 and 7 with the 2.1 release of the module. When I mouse over it, nothing happens, and IE throws an "object expected" error. When I use Microsoft's script debugger, it takes me to this line in my page source:

onmouseover="return addthis_open(this, '', '[URL]', '[TITLE]')"