my sqlite database is read only

i have a class that copy database from asset to /data/data

it works when i use select query. but when i use insert query return error that my database is read only my class code is :

  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();
}

}