OCRHMM with limitated vocabulary

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Hi to everyone i'm trying to use the OCRHMMDecoder on a video in order to recognize a char. I've take this code from the examples in the installation folder of opencv

string vocabulary = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
Ptr<OCRHMMDecoder::ClassifierCallback> ocr = loadOCRHMMClassifierCNN("/home/fed27/opencv_contrib/modules/text/samples/OCRBeamSearch_CNN_model_data.xml.gz");
 ocr->eval(mask, out_classes, out_confidences);
 output=vocabulary[out_classes[i]];
 string s = QString::fromStdString(output) ;
 qWarning() << " OCR_Tesseract  output "<< s ;

it works well but i need the vocabulary limited to few char in order to have more possibilities to guess the right one. If i try to reduce the vocabulary string it gives an error.

thanks you.