[jQuery] toLowerCase method works in Google Chrome but not IE

Hi,

Code snippet is as follows:

function setStartMood(cookiemood) {

if(cookiemood) {
$("#moodlist").val(cookiemood);
$("#plsselectmood").css( {'visibility': 'hidden'} );
mood = cookiemood.toLowerCase();
}
else {
$("#plsselectmood").css( {'visibility': 'visible'} );
}
return mood;
}

The function works perfectly in Chrome and Firefox but IE gives the
ff. error message (object doesn't support this property or method),
referring to the toLowerCase method. IE is really annoying!