How to pass a null variable to a SQL Server stored procedure from C# / .NET code

I'm calling a SQL Server stored procedure from a piece of C# / .NET code like this:

SqlHelper.ExecuteDataset(sqlConnection, CommandType.StoredProcedure, STORED_PROC_NAME, sqlParameters);

where the sqlParameters variable is defined as:

SqlParameter[] sqlParameters = new SqlParameter[SQL_NUMBER_PARAMETERS];

Log.Logger.Debug(string.Format("Running proc: {0} ", STORED_PROC_NAME));

SqlParameters[0] = new SqlParameter("fieldID", SqlDbType.BigInt );
SqlParameters[0].Value = fieldID;
SqlParameters[0].Direction = ParameterDirection.Input;

I need to now pass in another two parameters to this stored procedure, both are of type SqlDateTime, which are going to be NULL in this case.