the parameterized query which was not supplied

This error will raise while the passing parameters with NULL values, we can handle this exception 

if(LoanRefNo!=null)

{

 parameters.Add("@lRefNo", SqlDbType.NVarChar, LoanRefNo, 50);

}

else{

  parameters.Add("@lRefNo", SqlDbType.NVarChar,DBNull.Value, 50);

}