TypeError :Error #1085: at flash.external::ExternalInterface$/call()

Hi...
I am calling javascript function of wrapper from flex. The funcation is called but when it return a value , the value is not returned to flex.
The same value if i print using alert in javascript its value is printed.

my javascript code is like this...

function readCookie() {
var nameEQ = "user=";
if(document.cookie!=""){
var ca = document.cookie.split(';');
for(var i=0;i < ca.length;i++) {
var c = ca;
var decript_data="";

while (c.charAt(0)==' ') c = c.substring(1,c.length);
if (c.indexOf(nameEQ) == 0) {
var temp = c.substring(nameEQ.length,c.length);
alert("temp="+temp);
decript_data=des("validation",temp,0,1,"name",0);
alert("user="+ decript_data); // it prints value of decript_data
return decript_data;
}
}
}
return null;
}


i am calling this function from flex as;

private function userSession():void{
var usrData:String;
usrData=ExternalInterface.call("readCookie");
}


when i run the application , i am getting this error:
TypeError: Error #1085: The element type "string" must be terminated by the matching end-tag "</string>".
at flash.external::ExternalInterface$/call()
at builds/userSession()
at builds/LoadFilterVal()
at builds/___Application1_creationComplete()
at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at mx.core::UIComponent/dispatchEvent()
at mx.core::UIComponent/set initialized()
at mx.managers::LayoutManager/doPhasedInstantiation()
at Function/http://adobe.com/AS3/2006/builtin::apply()
at mx.core::UIComponent/callLaterDispatcher2()
at mx.core::UIComponent/callLaterDispatcher()


..returning a simple string from javascript is returned well to flex , but wen a variable containing string is returned the above error occurs..

Please any one can help me to solve this problem.