check

book.tex

@@ -2205,7 +2205,11 @@ to represent the environment.
 %
 The \code{interp\_exp} function takes the current environment,
 \code{env}, as an extra parameter.  When the interpreter encounters a
-variable, it looks up the corresponding value in the dictionary.
+variable, it looks up the corresponding value in the dictionary. If
+the variable is not in the dictionary (because the variable was not
+defined) then the dictionary lookup will fail and the interpreter will
+halt with an error. Recall that the compiler is not obligated to
+compile such programs (Section~\ref{sec:interp_Lint}).
 %
 \racket{When the interpreter encounters a \key{Let}, it evaluates the
   initializing expression, extends the environment with the result