SQLite database is read only

I have some code that copies a database from asset to /data/data.

When I use SELECT query, it works; but when I use INSERT query, it returns an error saying that my database is read only.

My code:

public class AssetDatabaseOpenHelper {

    private static final String DB_NAME = "database";

    private Context context;

    public AssetDatabaseOpenHelper(Context context) {
        this.context = context;
    }

    public SQLiteDatabase openDatabase() {
        File dbFile = context.getDatabasePath(DB_NAME);

        if (!dbFile.exists()) {
            try {
                copyDatabase(dbFile);
            } catch (IOException e) {
                throw new RuntimeException("Error creating source database", e);
            }
        }

        return SQLiteDatabase.openDatabase(dbFile.getPath(), null, SQLiteDatabase.OPEN_READONLY);
    }

    private void copyDatabase(File dbFile) throws IOException {
        InputStream is = context.getAssets().open(DB_NAME);
        OutputStream os = new FileOutputStream(dbFile);

        byte[] buffer = new byte[1024];
        while (is.read(buffer) > 0) {
            os.write(buffer);
        }

        os.flush();
        os.close();
        is.close();
    }
}

Code using my class in my activity:

AssetDatabaseOpenHelper adb = new AssetDatabaseOpenHelper(context);
SQLiteDatabase db = adb.openDatabase();