Access :: Insert Query Due To Database Relations

i have a problem executing an insert query. whenever i try to insert i have an exeption telling me it cant execute because a record needed in another table; where i select a value form that other table according a listbox, and then insert it into this table. here is a sample of my code:

OleDbConnection conn = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=|DataDirectory|emp2.mdb");

int a = 0;
conn.Open();
string sqlcmd = "select id from nationality where descreption = '" + DropDownList1.Text + "'";
OleDbCommand readNatCmd = new OleDbCommand(sqlcmd, conn);
OleDbDataReader readerNat = readNatCmd.ExecuteReader();
while (readerNat.Read())
{
a = readerNat.GetInt32(0);
}
readerNat.Close();
conn.Close();
string saveEmpcmd = @"insert into employee " + "(nationality) " + "values (@nationality)";
OleDbCommand objCmd = new OleDbCommand(saveEmpcmd, conn);
objCmd.Parameters.AddWithValue("@nationality", a);
conn.Open();
objCmd.ExecuteNonQuery();
conn.Close();

now, the form executes with no errors, but it catches an exeption caused by the ExecuteNonQuery funtion; here it is in details:

System.Data.OleDb.OleDbException: You cannot add or change a record because a related record is required in table 'nationality'. at System.Data.OleDb.OleDbCommand.ExecuteCommandTextErrorHandling(OleDbHResult hr) at System.Data.OleDb.OleDbCommand.ExecuteCommandTextForSingleResult(tagDBPARAMS
dbParams, Object& executeResult) at System.Data.OleDb.OleDbCommand.ExecuteCommandText(Object& executeResult) at System.Data.OleDb.OleDbCommand.ExecuteCommand(CommandBehavior behavior, Object& executeResult) at System.Data.OleDb.OleDbCommand.ExecuteReaderInternal(CommandBehavior
behavior, String method) at System.Data.OleDb.OleDbCommand.ExecuteNonQuery() at Default2.Button1_Click(Object sender, EventArgs e) in c:Documents and SettingsAdministratorMy DocumentsVisual Studio 2005WebSitesempDefault2.aspx.cs:line 305

when i remove the relation between the tables, the exeption isnt catched anymore, but the value stored in the employee table is 0 how can i solve this?