diff --git a/interp.py b/interp.py
index 94f56e0..aeab99e 100644
--- a/interp.py
+++ b/interp.py
@@ -37,7 +37,10 @@
 
     def resolve_value(self, value):
         if value.__class__ == ast_types.Reference:
-            return self.env[value.value]
+            if value.value in self.env:
+                return self.env[value.value]
+            else:
+                raise InterpreterError("Unknown environment value %s" % (value.value))
         elif value.__class__ == ast_types.Text:
             return Text(value.value)
         else: