diff --git a/Doc/lib/libstring.tex b/Doc/lib/libstring.tex
index 3939229..7d6f4bd 100644
--- a/Doc/lib/libstring.tex
+++ b/Doc/lib/libstring.tex
@@ -144,6 +144,11 @@
 \function{translate()} or \function{regex.compile()}, that will map
 each character in \var{from} into the character at the same position
 in \var{to}; \var{from} and \var{to} must have the same length. 
+
+\strong{Warning:} don't use strings derived from \code{lowercase} and
+\code{uppercase} as arguments; in some locales, these don't have the
+same length.  For case conversions, always use \function{lower()} and
+\function{upper()}.
 \end{funcdesc}
 
 \begin{funcdesc}{split}{s\optional{, sep\optional{, maxsplit}}}