diff --git a/graphene/types/union.py b/graphene/types/union.py
index fa178594..622f465e 100644
--- a/graphene/types/union.py
+++ b/graphene/types/union.py
@@ -42,4 +42,4 @@ class Union(six.with_metaclass(UnionMeta)):
     resolve_type = None
 
     def __init__(self, *args, **kwargs):
-        raise Exception("An Union cannot be intitialized")
+        raise Exception("A Union cannot be intitialized")