diff --git a/ogc/na/annotate_schema.py b/ogc/na/annotate_schema.py
index 74f39e9..b678d61 100644
--- a/ogc/na/annotate_schema.py
+++ b/ogc/na/annotate_schema.py
@@ -148,7 +148,12 @@
 
 CURIE_TERMS = '@id', '@type', '@index'
 
-UNDEFINED = object()
+class Undefined:
+
+    def __bool__(self):
+        return False
+
+UNDEFINED = Undefined()
 
 context_term_cache = LRUCache(maxsize=20)
 requests_session = requests_cache.CachedSession('ogc.na.annotate_schema', backend='memory', expire_after=180)