Only MySqlParameter objects may be stored

Question

Hi all I have a written a sample code to insert in to mysql database using stored procedure along with entity framework as follows

MySqlParameter userName = new MySqlParameter("_UserName", MySqlDbType.VarChar);

                userName.Value = txtUserName.Text;

                MySqlParameter password = new MySqlParameter("_Password", MySqlDbType.VarChar);

                password.Value = txtPassword.Text;

                MySqlParameter FirstName = new MySqlParameter("_FirstName", MySqlDbType.VarChar);

                FirstName.Value = txtFirstName.Text;

                MySqlParameter LastName = new MySqlParameter("_LastName", MySqlDbType.VarChar);

                LastName.Value = txtLastName.Text;

                entities.ExecuteStoreCommand("uspInsertUsers _UserName,_Password,_FirstName,_LastName", userName,password,FirstName,LastName);

But I am getting an exception as Only MySqlParameter objects may be stored can some one help me..