I am trying to add auto complete for *.lisp files. My slime setting is:

(add-to-list 'load-path "~/.emacs.d/plugins/slime/")
(setq slime-lisp-implementations
      '((sbcl ("/opt/sbcl/bin/sbcl" "--core" "/opt/sbcl/lib/sbcl/sbcl.core")
              :coding-system utf-8-unix
              :env ("SBCL_HOME=/opt/sbcl/lib/sbcl"))
        (ccl ("/opt/ccl/lx86cl64")
             :coding-system utf-8-unix)))
(require 'slime-autoloads)
(slime-setup '(slime-fancy))

And ac-slime setting is:

(require 'ac-slime)
(add-hook 'slime-mode-hook 'set-up-slime-ac)
(add-hook 'slime-repl-mode-hook 'set-up-slime-ac)
(eval-after-load "auto-complete"
  '(add-to-list 'ac-modes 'slime-repl-mode))

Each time I type a word in *.lisp file, auto complete popups some candidates but after a second minibuffer outputs

error in process filter: Reply to canceled synchronous eval request 
tag=slime-result-6-19579 
sexp=(swank:simple-completions "de" (quote "COMMON-LISP-USER"))

and the popup stuck for a while. After that I can continue my selection.

My question is how to remove this error and stuck? Any help is appreciated.