[CM] adding an extra channel

[Bill Schottstaedt]

I think in your code you have a line like

  (for-each free-sample-reader readers)

which applies free-sample-reader to each member of the list readers.
Since this list can now have #f as well, that line needs to be

  (for-each (lambda (r) (if r (free-sample-reader r))) readers)

or some reasonable facsimile thereof.