--- ../tests/test__collection_api.py	2025-10-07 23:58:44.191678100 +0300
+++ ../tests/test__collection_api.py	2025-10-07 23:59:01.850191288 +0300
@@ -5282,7 +5282,7 @@
             collection.insert_one({'a': {'b'}})
         if version.parse(pymongo.version) < version.parse('3.8'):
             return
-        self.assertEqual(str(cm.exception), "cannot encode object: {'b'}, of type: <class 'set'>")
+        self.assertIn("cannot encode object: {'b'}, of type: <class 'set'>", str(cm.exception))
 
     @skipIf(not helpers.HAVE_PYMONGO, 'pymongo not installed')
     def test_insert_bson_invalid_encode_type(self):
--- ../tests/test__mongomock.py	2025-10-08 00:07:35.821805227 +0300
+++ ../tests/test__mongomock.py	2025-10-08 00:12:50.684290193 +0300
@@ -683,12 +683,17 @@
         }})
         self.cmp.compare_ignore_order.find({'name': {
             '$regex': upper_regex,
-            '$options': 'I',
-        }})
-        self.cmp.compare_ignore_order.find({'name': {
-            '$regex': upper_regex,
-            '$options': 'z',
+            '$options': 'i',
         }})
+        with self.assertRaises(OperationFailure) as cm:
+            self.cmp.compare_ignore_order.find(
+                {
+                    'name': {
+                        '$regex': upper_regex,
+                        '$options': 'z',
+                    }
+        })
+        self.assertIn('invalid flag', str(cm.exception))
 
     def test__find_by_regex_string(self):
         """Test searching with regular expression string."""
@@ -700,8 +705,9 @@
         self.cmp.compare_ignore_order.find({'name': {'$regex': 'bob|sam'}})
         self.cmp.compare_ignore_order.find({'name': {'$regex': 'bob|notsam'}})
         self.cmp.compare_ignore_order.find({'name': {'$regex': 'Bob', '$options': 'i'}})
-        self.cmp.compare_ignore_order.find({'name': {'$regex': 'Bob', '$options': 'I'}})
-        self.cmp.compare_ignore_order.find({'name': {'$regex': 'Bob', '$options': 'z'}})
+        self.cmp.compare_ignore_order.find({'name': {'$regex': 'Bob', '$options': 'i'}})
+        with self.assertRaises(OperationFailure):
+            self.cmp.compare_ignore_order.find({'name': {'$regex': 'Bob', '$options': 'z'}})
 
     def test__find_in_array_by_regex_object(self):
         """Test searching inside array with regular expression object."""