JS entity problem

Hi all.

The entity from the code below couse an error and fails to display any 
code. But when using the last code everything is OK. How to tell parser 
that he is parsing XHTML1.1 code to let him know all its entities?

here's JSConsoleError:

	Error: [Exception... "An invalid or illegal string was specified" 
code: "12" nsresult: "0x8053000c (NS_ERROR_DOM_SYNTAX_ERR)"

	XML Parsing Error: undefined entity
	Location:
	Line Number 1, Column 501:

here's JS code:

	var displayDiv = document.getElementById('displayDiv');
	displayDiv.innerHTML = '<p>dfdsfdsfdsf&isin;</p>';

here's XHTML code

	<div id="displayDiv" />

and fallowing JS code works ok:

	var displayDiv = document.getElementById('displayDiv');
	displayDiv.innerHTML = 'dfdsfdsfdsf&isin;';

Regards Peter