diff --git a/lisp/faces.el b/lisp/faces.el
index 3788606e25b..74fb666f6cc 100644
--- a/lisp/faces.el
+++ b/lisp/faces.el
@@ -408,7 +408,8 @@ Use `face-attribute' for finer control."
 
 (defun set-face-documentation (face string)
   "Set the documentation string for FACE to STRING."
-  (put face 'face-documentation string))
+  ;; The text doesn't get into DOC.
+  (put face 'face-documentation (purecopy string)))
 
 
 (defalias 'face-doc-string 'face-documentation)