diff --git a/Doc/lib/libstdtypes.tex b/Doc/lib/libstdtypes.tex
index ae1ba43..01e800a 100644
--- a/Doc/lib/libstdtypes.tex
+++ b/Doc/lib/libstdtypes.tex
@@ -1368,7 +1368,7 @@
 
 \item[(5)] \function{setdefault()} is like \function{get()}, except
 that if \var{k} is missing, \var{x} is both returned and inserted into
-the dictionary as the value of \var{k}.
+the dictionary as the value of \var{k}. \var{x} defaults to \var{None}.
 
 \item[(6)] \function{popitem()} is useful to destructively iterate
 over a dictionary, as often used in set algorithms.