mootools 1.2 causes JS error in IE9

Question

I have the following code in java-script to create a couple of nice Calendar controls on an .aspx page.

var calFrom = new Calendar({ rangeDateStart: 'd/m/Y' }, { classes: ['dashboard'], direction: .5, days: shortDayName, months: shortMonthName }); 

Error is "SCRIPT5022: DOM Exception: INVALID_CHARACTER_ERR (5)" this is occurring in the mootools document.implement method and works fine in IE8.

Anyone any ideas how to get round? I upgraded to mootools 1.2.6 but causes an infinite loop, think because of how my pages are hanging together.

Cheers