A potentially dangerous Request.Form value was detected from the client (arg="<lnapp><...

Hi guys,

i developed a service it is working fine when i am using in another application.
but when i test it like this : 

Solution File : rt click on Service file -> open in browser

so we can see the webmethods exposed to us.
when i want to test my webmethod that takes string as argument 
so my string is like this  :  
wen i pass above string it is giving follwing error
A potentially dangerous Request.Form value was detected from the client (arg="<lnapp></lnapp>").

can anyone help on this