As reported on the mailing list:

Hey guys,

I was wondering if the whole hier_block2 thing was working yet.  I tried to
run the blks2.synthesis_filterbank in a manner that seemed appropriate and
got the following error:

terminate called after throwing an instance of 'std::runtime_error'
  what():  streams_to_stream(5): insufficient connected output ports (2
needed, 1 connected)

Is this a bug in the filterbank code, or does this have to do with
hier_block2 not working yet?

Any help would be greatly appreciated.  Thanks!

-Ben