diff --git a/src/calibre/ebooks/metadata/__init__.py b/src/calibre/ebooks/metadata/__init__.py
index 55d46a44c2..1da348443f 100644
--- a/src/calibre/ebooks/metadata/__init__.py
+++ b/src/calibre/ebooks/metadata/__init__.py
@@ -97,7 +97,8 @@ def author_to_author_sort(
 
     author_use_surname_prefixes = tweaks['author_use_surname_prefixes'] if use_surname_prefixes is None else use_surname_prefixes
     if author_use_surname_prefixes:
-        author_surname_prefixes = frozenset(x.lower() for x in (tweaks['author_surname_prefixes'] if surname_prefixes is None else surname_prefixes))
+        author_surname_prefixes = frozenset(
+            force_unicode(x).lower() for x in (tweaks['author_surname_prefixes'] if surname_prefixes is None else surname_prefixes))
         if len(tokens) == 2 and tokens[0].lower() in author_surname_prefixes:
             return author
 
diff --git a/src/calibre/ebooks/metadata/test_author_sort.py b/src/calibre/ebooks/metadata/test_author_sort.py
index 29e79eda78..9f9d366c69 100644
--- a/src/calibre/ebooks/metadata/test_author_sort.py
+++ b/src/calibre/ebooks/metadata/test_author_sort.py
@@ -102,6 +102,9 @@ class TestAuthorToAuthorSort(unittest.TestCase):
             self.check_all_methods('Leonardo Da Vinci',
                                     invert='Da Vinci, Leonardo',
                                     nocomma='Da Vinci Leonardo')
+            self.check_all_methods('Liam Da Mathúna',
+                                    invert='Da Mathúna, Liam',
+                                    nocomma='Da Mathúna Liam')
             self.check_all_methods('Van Gogh')
             self.check_all_methods('Van')
         with Tweak('author_use_surname_prefixes', False):