UI Position, target.offset() returns object with read only properties. Position attempts to change these and returns exception in Firefox 3.6

In some cases, Position uses object that has read only properties. Here are the code excerpts:

        } else {
		targetWidth = target.outerWidth();
		targetHeight = target.outerHeight();
		basePosition = target.offset();
	}

target.offset() is browser object with read only properties.

        if ( options.at[1] === "bottom" ) {
		basePosition.top += targetHeight;
	} else if ( options.at[1] === center ) {
		basePosition.top += targetHeight / 2;
	}

Exception gets thrown at basePosition.top += targetHeight; because it cant assign value to read only property.

Dirty solution i used:

basePosition = $.extend({},target.offset());