Save a numpy sparse matrix into file

I want to save the result of TfidfVectorizer in sklearn.feature_extraction.text into a text file for future use. As I found, it is a sparse matrix of type ''. However when I try to save it using the following code

np.savetxt('Feature_TfIdf.txt', X_Tfidf, fmt='%2.6f')

I get an error like this

IndexError: tuple index out of range