This line (usocket:wait-for-input some-client-socket :timeout 0 :ready-only t) Causes a condition "CDR: :INPUT is not a list." It occurs regardless of whether some-client-socket is in a list or not. The same issue doesn't happen when working with a socket returned by socket-listen, only when some-client-socket is a socket created by socket-connect or socket-accept. Backtrace here: http://pastebin.com/zjT8ghDA If anyone finds a relatively simple fix, but it would take a while to get it updated in quicklisp, a response would be appreciated. Thanks for the library! - Reepca