如何在文件的一部分使用DeflateStream ?

I'm working on a solution to my other question which is reading the data in the 'zTXt' chunks of a PNG. I am as far as locating the chunks in the file, and reading the zTXt's keyword. I'm having trouble reading the compressed portion of zTXt. I've never worked with the DeflateStream object before, and am having some trouble with it. When reading, it appears to expect the length parameter to be in 'uncompressed' bytes. In my case however, I only know the length of the data in 'compressed' bytes. To hopefully get around this, I put all the data that needed to be decompressed into a MemoryStream, and then 'read to end' with a DeflateStream. Now that's just peachy, except it throws an InvalidDataException with the message "Block length does not match with its complement." Now I have no idea what this means. What could be going wrong?