accordion/dimensions - Object doesn't support this property or method in IE6/7

I've been using jquery 2.1, ui.accordion.js and jquery.dimensions.js - everything works fine in firefox, but in IE6/7 I get

Object doesn't support this property or method on line 169 when I click on an accordion tab - the tab moves but there's a lot of flicker. I'm assuming this is referring to jquery.dimensions.js, in which case it's the show part of:

$.ui.accordion.animations[settings.animated]({

toShow: toShow,

toHide: toHide,

finished: finished,

down: down

});

I'm using this to define the accordion (unordered list):

$(document).ready(function(){

$("#flags") .accordion({

header: 'a.language',

event: 'click'

});

});