When setting width or height on an element, if size is null or NaN, it is rendered to a string and appended with 'px', such as 'nullpx' or 'NaNpx'. In Internet Explorer this causes errors to be thrown.

While it's not jQuery's fault that the bad data has been given to the height() or width() functions, this should be handled more elegantly than performing generic string conversion to anything that's not a string.