MAD doesn't decode last frame in stream buffer unless it has 8 bytes extra

Pass a buffer to mad_stream_buffer which ends exactly on a frame boundary. 
mad_frame_decode will fail on the last frame and set the error to 
MAD_ERROR_BUFLEN even though entire frame is there. As much as 7 extra 
bytes can be added to the end of the buffer and the last frame decode will 
still fail. If 8 bytes are added, the frame decode will succeed.

Is this a bug or by design? If by design, how can I decode the last frame 
without adding padding after it?