diff --git a/loxi_ir/ir.py b/loxi_ir/ir.py
index 21f760c..82227fb 100644
--- a/loxi_ir/ir.py
+++ b/loxi_ir/ir.py
@@ -172,7 +172,7 @@
         if self.is_fixed_length:
             return self.base_length
         else:
-            raise Exception("Not a fixed length class: {}".self.name)
+            raise Exception("Not a fixed length class: {}".format(self.name))
 
 """ one class unified across openflow versions. Keeps around a map version->versioned_class """
 class OFUnifiedClass(OFClass):