See http://css3pie.com/forum/viewtopic.php?f=3&t=357 -- when using a length with units other than px, PIE tries to insert the length-calc element into the target element. This causes an error for some elements which cannot accept children, e.g. input[type=submit].
Also happens with table elements, though that does not throw an error, it just results in the length being calculated as 0px. See http://css3pie.com/forum/viewtopic.php?f=3&t=573
When calculating em unit size on elements that don't accept children,…
… insert the length calc element into the parent and perform an additional calculation based on the specified font-size. Prevents error trying to insert into the childless element. Fixes #99.