diff --git a/src/tufup/client.py b/src/tufup/client.py
index d73cf25..b1974c7 100644
--- a/src/tufup/client.py
+++ b/src/tufup/client.py
@@ -153,7 +153,11 @@ def check_for_updates(
 
         If `patch` is `False`, a full update is enforced.
         """
-        included = {None: '', '': '', 'a': 'abrc', 'b': 'brc', 'rc': 'rc'}
+        # invalid pre-release specifiers are ignored, with a warning
+        pre_map = dict(a='abrc', b='brc', rc='rc')
+        prereleases = pre_map.get(pre, '')
+        if pre and not prereleases:
+            logger.warning(f'ignoring invalid pre-release specifier: "{pre}"')
         # refresh top-level metadata (root -> timestamp -> snapshot -> targets)
         try:
             self.refresh()
@@ -177,7 +181,7 @@ def check_for_updates(
             item
             for item in all_new_targets.items()
             if item[0].is_archive
-            and (not item[0].version.pre or item[0].version.pre[0] in included[pre])
+            and (not item[0].version.pre or item[0].version.pre[0] in prereleases)
         )
         new_archive_meta = None
         if new_archives:
diff --git a/tests/test_client.py b/tests/test_client.py
index 39922fd..a2e8c15 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -150,16 +150,18 @@ def test_download_and_apply_update(self):
 
     def test_check_for_updates(self):
         # expectations (based on targets in tests/data/repository):
-        # - pre=None: only full releases are included, so finds 2.0 patch
+        # - pre=None, '', or 'invalid': only full releases are included, finds 2.0 patch
         # - pre='a': finds all, but total patch size exceeds archive size
         # - pre='b': there is no 'b' release, so this finds same as 'rc'
         # - pre='rc': finds 2.0 and 3.0rc0, total patch size smaller than archive
         client = self.get_refreshed_client()
         with patch.object(client, 'refresh', Mock()):
-            for pre, expected in [(None, 1), ('a', 1), ('b', 2), ('rc', 2)]:
+            for pre, expected in [
+                (None, 1), ('', 1), ('a', 1), ('b', 2), ('rc', 2), ('invalid', 1)
+            ]:
                 with self.subTest(msg=pre):
                     target_meta = client.check_for_updates(pre=pre)
-                    self.assertTrue(target_meta)
+                    self.assertTrue(expected and target_meta)
                     self.assertEqual(expected, len(client.new_targets))
                     if pre == 'a':
                         self.assertTrue(