1 files changed, 4 insertions, 1 deletions
diff --git a/python/step2_eval.py b/python/step2_eval.py
index bb5d6f8..65972a2 100644
--- a/python/step2_eval.py
+++ b/python/step2_eval.py
@@ -12,7 +12,10 @@ def READ(str):
 # eval
 def eval_ast(ast, env):
     if symbol_Q(ast):
-        return env[ast]
+        try:
+            return env[ast]
+        except:
+            raise Exception("'" + ast + "' not found")
     elif list_Q(ast):
         return new_list(*map(lambda a: EVAL(a, env), ast))
     elif vector_Q(ast):