diff --git a/Doc/lib/libweakref.tex b/Doc/lib/libweakref.tex
index 12a1220..fc88916 100644
--- a/Doc/lib/libweakref.tex
+++ b/Doc/lib/libweakref.tex
@@ -58,7 +58,8 @@
   retrieved by calling the reference object if the referent is still
   alive; if the referent is no longer alive, calling the reference
   object will cause \code{None} to be returned.  If \var{callback} is
-  provided, it will be called when the object is about to be
+  provided and not \constant{None},
+  it will be called when the object is about to be
   finalized; the weak reference object will be passed as the only
   parameter to the callback; the referent will no longer be available.