diff --git a/test/test_geneve.py b/test/test_geneve.py
index ebe95a0..4d6e3f5 100644
--- a/test/test_geneve.py
+++ b/test/test_geneve.py
@@ -113,7 +113,7 @@
                 remote_address=cls.mcast_ip4n, mcast_sw_if_index=1,
                 is_add=is_add, vni=vni)
             if r.sw_if_index == 0xffffffff:
-                raise "bad sw_if_index"
+                raise ValueError("bad sw_if_index: ~0")
 
     @classmethod
     def add_shared_mcast_dst_load(cls):