how must I align buffers passed to mad_stream_buffer?

My decoder uses an arbitrary buffer length and passes data blocks of this 
size, read from the MP3 file, to mad_stream_buffer. However, MAD throws a 
"lost synchronization" error immediately after returning from the input 
callback which feeds the data to mad_stream_buffer. The resulting decoded 
audio has artifacts at the equivelant locations.

Clearly mad expects the MP3 data to be aligned, perhaps on frame 
boundaries, but I can't find any mention of this in the source comments.