SqlCeCommand 可为空的值

我的代码：

SqlCeConnection sql = new SqlCeConnection(@"Data Source=C:'DB.sdf");
sql.Open();
cmd = new SqlCeCommand("INSERT INTO xxx(aaa) VALUES(@aaa)", sql);
String param = null;  //doesn't work
//String param = "blah" //works
cmd.Parameters.AddWithValue("@aaa", param);
cmd.ExecuteNonQuery(); //(1)
sql.Close();

（1） 当 param 为 null 时引发异常。数据库允许列aaa中的值为空。如何将空插入表 xxx ？

例外：
Parameterized query 'INSERT INTO xxx(aaa) VALUES(@aaa)' expects a parameter value which was not supplied.

var param = DBNull.Value;

var param = DBNull.Value;