SqlCeCommand NULL Values

My code is:

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) throws an exception if paramnull. The database allows a column aaavalue of zero. How to insert zero into table xxx?

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