Getting error while saving through Filestorage using opencv

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I am trying to save the vocabulary from the BOW algorithm , below is my code

FileStorage fs; 
fs.open("Vocabulary.xml", FileStorage::WRITE); 
Mat vocabulary = bow.cluster(); 
fs << vocabulary ;
fs.release();
FileStorage fs2; 
fs2.open("Vocabulary.xml", FileStorage::READ); 
fs2 ["Vocabulary"] >> vocabulary ;
dextract.setVocabulary(vocabulary);

cv::Mat training_mat(num_img , dictionarySize,CV_32FC1);
cv::Mat labels(num_img,1,CV_32FC1);

CvSVM svm;
svm.load( "trainsvm.xml" );

Here is my error :

OpenCV Error: Unspecified error (No element name has been given) in unknown function , file c:\opencv\2.4.4\build\include\opencv2\core\operations.hpp , line 2908