diff --git a/google/cloud/ndb/query.py b/google/cloud/ndb/query.py
index 6eba8bd3..161ea092 100644
--- a/google/cloud/ndb/query.py
+++ b/google/cloud/ndb/query.py
@@ -1392,7 +1392,13 @@ def __init__(
                 else:
                     project = ancestor.app()
                 if namespace is not None:
-                    if namespace != ancestor.namespace():
+                    # if namespace is the empty string, that means default
+                    # namespace, but after a put, if the ancestor is using
+                    # the default namespace, its namespace will be None,
+                    # so skip the test to avoid a false mismatch error.
+                    if namespace == "" and ancestor.namespace() is None:
+                        pass
+                    elif namespace != ancestor.namespace():
                         raise TypeError("ancestor/namespace mismatch")
                 else:
                     namespace = ancestor.namespace()
diff --git a/tests/system/test_query.py b/tests/system/test_query.py
index 96838206..7e57f139 100644
--- a/tests/system/test_query.py
+++ b/tests/system/test_query.py
@@ -161,6 +161,56 @@ class SomeKind(ndb.Model):
     assert [entity.foo for entity in results] == [-1, 0, 1, 2, 3, 4]
 
 
+def test_ancestor_query_with_namespace(client_context, dispose_of, other_namespace):
+    class Dummy(ndb.Model):
+        foo = ndb.StringProperty(default="")
+
+    entity1 = Dummy(foo="bar", namespace="xyz")
+    parent_key = entity1.put()
+    dispose_of(entity1.key._key)
+
+    entity2 = Dummy(foo="child", parent=parent_key, namespace=None)
+    entity2.put()
+    dispose_of(entity2.key._key)
+
+    entity3 = Dummy(foo="childless", namespace="xyz")
+    entity3.put()
+    dispose_of(entity3.key._key)
+
+    with client_context.new(namespace=other_namespace).use():
+        query = Dummy.query(ancestor=parent_key, namespace="xyz")
+        results = eventually(query.fetch, length_equals(2))
+
+    assert results[0].foo == "bar"
+    assert results[1].foo == "child"
+
+
+def test_ancestor_query_with_default_namespace(
+    client_context, dispose_of, other_namespace
+):
+    class Dummy(ndb.Model):
+        foo = ndb.StringProperty(default="")
+
+    entity1 = Dummy(foo="bar", namespace="")
+    parent_key = entity1.put()
+    dispose_of(entity1.key._key)
+
+    entity2 = Dummy(foo="child", parent=parent_key, namespace=None)
+    entity2.put()
+    dispose_of(entity2.key._key)
+
+    entity3 = Dummy(foo="childless", namespace="")
+    entity3.put()
+    dispose_of(entity3.key._key)
+
+    with client_context.new(namespace=other_namespace).use():
+        query = Dummy.query(ancestor=parent_key, namespace="")
+        results = eventually(query.fetch, length_equals(2))
+
+    assert results[0].foo == "bar"
+    assert results[1].foo == "child"
+
+
 @pytest.mark.usefixtures("client_context")
 def test_projection(ds_entity):
     entity_id = test_utils.system.unique_resource_id()
diff --git a/tests/unit/test_query.py b/tests/unit/test_query.py
index 7e4966a2..672bce7a 100644
--- a/tests/unit/test_query.py
+++ b/tests/unit/test_query.py
@@ -1244,6 +1244,13 @@ def test_constructor_with_ancestor_and_namespace():
         query = query_module.Query(ancestor=key, namespace="space")
         assert query.namespace == "space"
 
+    @staticmethod
+    @pytest.mark.usefixtures("in_context")
+    def test_constructor_with_ancestor_and_default_namespace():
+        key = key_module.Key("a", "b", namespace=None)
+        query = query_module.Query(ancestor=key, namespace="")
+        assert query.namespace == ""
+
     @staticmethod
     @pytest.mark.usefixtures("in_context")
     def test_constructor_with_ancestor_parameterized_thing():