How to detect only one character in Firebase ML-Kit?

I set up ML-Kit like AndroidExample and using on-device recognition mode. It's working so well. but If we have a single character like 'A','5','K','9' it can't recognize anything! it only works for more than one length of string! I need to recognize only one character. What am I missing?!

This is my Function that gets a bitmap of an image and finds the texts from it.

private void RecognizeText(Bitmap bitmap) {
    FirebaseVisionImage image = FirebaseVisionImage.fromBitmap(bitmap);
    FirebaseVisionTextRecognizer detector = FirebaseVision.getInstance()
        .getOnDeviceTextRecognizer();

    Task<FirebaseVisionText> result =
    detector.processImage(image)
        .addOnSuccessListener(new OnSuccessListener<FirebaseVisionText>() {
        @Override
        public void onSuccess(FirebaseVisionText firebaseVisionText) {
            String s = firebaseVisionText.getText() + " | " + firebaseVisionText.getText().length();
            Toast.makeText(MainActivity.this, s, Toast.LENGTH_LONG).show();

            textView.setText(s);
        }
    })
        .addOnFailureListener(
    new OnFailureListener() {
        @Override
        public void onFailure(Exception e) {
            Log.d("EEEEEEEEVVVVVV", e.toString());
        }
    });
}