diff --git a/chatmastermind/message.py b/chatmastermind/message.py
index 902aaa2..820d104 100644
--- a/chatmastermind/message.py
+++ b/chatmastermind/message.py
@@ -436,13 +436,14 @@ class Message():
         Filter tags based on their prefix (i. e. the tag starts with a given string)
         or some contained string.
         """
-        res_tags = self.tags
-        if res_tags:
-            if prefix and len(prefix) > 0:
-                res_tags -= {tag for tag in res_tags if not tag.startswith(prefix)}
-            if contain and len(contain) > 0:
-                res_tags -= {tag for tag in res_tags if contain not in tag}
-        return res_tags or set()
+        if not self.tags:
+            return set()
+        res_tags = self.tags.copy()
+        if prefix and len(prefix) > 0:
+            res_tags -= {tag for tag in res_tags if not tag.startswith(prefix)}
+        if contain and len(contain) > 0:
+            res_tags -= {tag for tag in res_tags if contain not in tag}
+        return res_tags
 
     def tags_str(self, prefix: Optional[str] = None, contain: Optional[str] = None) -> str:
         """
diff --git a/tests/test_message.py b/tests/test_message.py
index 83a73ea..2a9d0ff 100644
--- a/tests/test_message.py
+++ b/tests/test_message.py
@@ -746,3 +746,18 @@ class MessageTagsStrTestCase(CmmTestCase):
 
     def test_tags_str(self) -> None:
         self.assertEqual(self.message.tags_str(), f'{TagLine.prefix} tag1')
+
+
+class MessageFilterTagsTestCase(CmmTestCase):
+    def setUp(self) -> None:
+        self.message = Message(Question('This is a question.'),
+                               tags={Tag('atag1'), Tag('btag2')},
+                               file_path=pathlib.Path('/tmp/foo/bla'))
+
+    def test_filter_tags(self) -> None:
+        tags_all = self.message.filter_tags()
+        self.assertSetEqual(tags_all, {Tag('atag1'), Tag('btag2')})
+        tags_pref = self.message.filter_tags(prefix='a')
+        self.assertSetEqual(tags_pref, {Tag('atag1')})
+        tags_cont = self.message.filter_tags(contain='2')
+        self.assertSetEqual(tags_cont, {Tag('btag2')})