Jscript runtime error

I recently updated my theme and am now getting this error:

Breaking on Jscript runtime error – Object doesn’t support this property or method.

Referring to the following code:

//function to decide if mobile or not
function isMobile(){
header = $(‘.responsive #header’);

if($(window).width() < settings.switchWidth)
{
header.addClass(‘mobile_active’);
return true;
}
else
{
header.removeClass(‘mobile_active’);
return false;
}
}

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Specifically to the line: header = $(‘.responsive #header’);

This error is creating a serious problem in IE8. I am very new to developing and do not know how to fix it. Can anyone help please?? It’s holding up my launch. Thank you!!!!

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