Just an FYI (is trac open to edits for the public? I guess not), I managed to narrow down the problem here to this: http://abcl.org/trac/ticket/323 (defun test (stream) (declare ;; (type stream stream) (stream stream) ;; (optimize speed (safety 1) (debug 0)) ) (write-byte 1 stream)) (flex:with-output-to-sequence (s) (test s)) If you evaluate test (C-x C-e), the second form works. If test is compiled, (C-c C-c), then the second form fails. Some sort of type check error? The flexi-streams *is* a stream: (block a (flex:with-output-to-sequence (s) (return-from a (list (type-of s) (typep s 'stream))))) => (FLEXI-STREAMS::VECTOR-OUTPUT-STREAM T) Yong