interp_exp needs an empty env

book.tex

@@ -2038,7 +2038,7 @@ call it \code{e0}, by creating an object of the \LangVar{} class
 and calling the \code{interp\_exp} method.
 {\if\edition\racketEd
 \begin{lstlisting}
-(send (new interp_Lvar_class) interp_exp e0)
+((send (new interp_Lvar_class) interp_exp '()) e0)
 \end{lstlisting}
 \fi}
 {\if\edition\pythonEd