[lxml] attributes() in DTD - possible bug?

Hello, I'm trying to iterate over all attributes in DTD. Here is a log from REPL: http://pastebin.com/qfSqYjCB attributes() is empty when using DTD from internalDTD, but they are definitely used (default values for attributes are properly set). Also, when using etree.DTD attributes are properly listed. Is this a bug or there is something that I have missed? P.S.: externalDTD is empty, and this seems right to me.