__doPostBack works in IE7 but not in IE6?

My __doPostBack event works fine in IE7, but not in IE6.  Any idea why?

Here's my code behind:

Page.ClientScript.GetPostBackEventReference(this, "ID=" + HttpUtility.HtmlAttributeEncode(ID)) + ";" 
And here's what it puts in the HTML:
 
__doPostBack('ctl00$SPWebPartManager1$g_bcaeb313_7384_4f7a_9224_0374c9ff07f5$ctl01','ID=1');   ...