Alexandria (QL version) doesn't compile on ECL and warns on CLISP because invalid LOOP forms are used in (at least) COPY-STREAM. EG: (loop :while (< input-position start) :for bytes-read = (read-sequence buffer input :end (min (length buffer) (- start input-position))) :do (progn (when (zerop bytes-read) (error "Could not read enough bytes from the input to fulfill the START requirement in ~S" 'copy-stream)) (incf input-position bytes-read))) And I _think_ according to http://www.lispworks.com/documentation/HyperSpec/Body/m_loop.htm, the :WHILE expression can't precede the :FOR expression.