Sys.ParameterCountException: Parameter count mismatch.

I used AutocompleteExtender within update panel and used contextKey to pass additional parameter.

That time i received this error

 Sys.ParameterCountException: Parameter count mismatch.

I have set ScriptMode="Release" in ScriptManager. Default Value is Auto. 

It solved the my problem. You can also used auto for the same but you have to set When retail is set to true, ASP.NET disables certain configuration settings such as trace output, custom errors, and debug capabilities.I have just set retail = true.So you can use ScriptMode= Auto.