diff --git a/pyenv-mode.el b/pyenv-mode.el
index eb0d253..f750c60 100644
--- a/pyenv-mode.el
+++ b/pyenv-mode.el
@@ -37,6 +37,7 @@
     (when (pyenv-mode-version)
       (concat "Pyenv:" (pyenv-mode-version) " ")))
   "How `pyenv-mode' will indicate the current python version in the mode line."
+  :type '(string)
   :group 'pyenv)
 
 (defun pyenv-mode-version ()