diff --git a/firestore/google/cloud/firestore_v1/watch.py b/firestore/google/cloud/firestore_v1/watch.py
index 2dd80b69eb67..458a3a94780f 100644
--- a/firestore/google/cloud/firestore_v1/watch.py
+++ b/firestore/google/cloud/firestore_v1/watch.py
@@ -667,7 +667,7 @@ def modify_doc(new_document, updated_tree, updated_map):
         key = functools.cmp_to_key(self._comparator)
 
         # Deletes are sorted based on the order of the existing document.
-        delete_changes = sorted(delete_changes, key=key)
+        delete_changes = sorted(delete_changes)
         for name in delete_changes:
             change, updated_tree, updated_map = delete_doc(
                 name, updated_tree, updated_map
diff --git a/firestore/tests/unit/v1/test_watch.py b/firestore/tests/unit/v1/test_watch.py
index c4037d4a6ab4..afd88b813081 100644
--- a/firestore/tests/unit/v1/test_watch.py
+++ b/firestore/tests/unit/v1/test_watch.py
@@ -736,6 +736,28 @@ class DummyDoc(object):
         )
         self.assertEqual(updated_map, doc_map)  # no change
 
+    def test__compute_snapshot_deletes_w_real_comparator(self):
+        from google.cloud.firestore_v1.watch import WatchDocTree
+
+        doc_tree = WatchDocTree()
+
+        class DummyDoc(object):
+            update_time = mock.sentinel
+
+        deleted_doc_1 = DummyDoc()
+        deleted_doc_2 = DummyDoc()
+        doc_tree = doc_tree.insert(deleted_doc_1, None)
+        doc_tree = doc_tree.insert(deleted_doc_2, None)
+        doc_map = {"/deleted_1": deleted_doc_1, "/deleted_2": deleted_doc_2}
+        delete_changes = ["/deleted_1", "/deleted_2"]
+        add_changes = []
+        update_changes = []
+        inst = self._makeOne(comparator=object())
+        updated_tree, updated_map, applied_changes = inst._compute_snapshot(
+            doc_tree, doc_map, delete_changes, add_changes, update_changes
+        )
+        self.assertEqual(updated_map, {})
+
     def test__reset_docs(self):
         from google.cloud.firestore_v1.watch import ChangeType