>Probably an oversight. Do you want to add some tests?

That's WIP

> Because it's not an IO error at all. No I/O occurs. You are just using
the file wrongly (or the wrong file), hence the ValueError.

Then when you try to wrap a non-readable stream into a readable buffered stream (like BufferedRWPair), it should raise a value error as well, but currently it's rather:
"""if not reader.readable(): raise IOError('"reader" argument must be readable.')"""