Assign Prototype To Event

I am wondering why the following works, on IE6, but with an error : "Not
implemented".

function TEST(){}
TEST.prototype.Initialize = function()
{
var mImage = new Image();
var mDate = new Date();
var start = mDate.getTime();
mImage.onload = this.Alerting(start);//WORKS with ERROR "Not implemented"
//mImage.onload = function(){this.Alerting(start);}//ERROR "Object doesn't
support this property or method"
mImage.src = "winxp.gif";
}
TEST.prototype.Alerting = function(i_string){alert(i_string);}
var mTest = new TEST();
mTest.Initialize();

Can anybody guess why?