diff --git a/Doc/ref/ref6.tex b/Doc/ref/ref6.tex
index d61165f8fcc..7212071338d 100644
--- a/Doc/ref/ref6.tex
+++ b/Doc/ref/ref6.tex
@@ -86,6 +86,8 @@ is requested at compile time.  Note that it is unnecessary to include
 the source code for the expression that failed in the error message;
 it will be displayed as part of the stack trace.
 
+Assignments to \code{__debug__} are illegal.  The value for the
+built-in variable is determined when the interpreter starts.
 
 \section{Assignment statements \label{assignment}}