c# nullable twine error

private string? typeofcontract
{
  { relapse (string?)viewstate["typeofcontract"]; }
  set { viewstate["typeofcontract"] = value; }
}

later formula i this:

typeofcontract = request.querystring["type"];

i am removing following blunder during chapter typeofcontract line stating:

the form 'string' contingency non-nullable value form method use
parameter 't' general form method
'system.nullable<t>'

any ideas? basically, i wish certain "type" exists querystring before behaving an action.