diff --git a/interp.py b/interp.py
index 9cb2b38..94f56e0 100644
--- a/interp.py
+++ b/interp.py
@@ -50,7 +50,11 @@
         args = []
         for arg in ast.arguments:
             args.append(self.resolve_value(arg))
-        return getattr(subject, "verb_" + ast.verb)(args)
+        verb = getattr(subject, "verb_" + ast.verb, None)
+        if verb:
+            return verb(args)
+        else:
+            raise InterpreterError("Unknown verb %s for subject %s" % (ast.verb, subject.__class__.__name__))
 
     def run_set(self, ast):
         self.env[ast.subject] = self.run_statement(ast.statement)