diff --git a/swh/core/tests/test_utils.py b/swh/core/tests/test_utils.py
--- a/swh/core/tests/test_utils.py
+++ b/swh/core/tests/test_utils.py
@@ -119,3 +119,15 @@
     actual_commonname2 = utils.commonname(b"/some/where/to/", b"/some/where/to/go/to")
     # then
     assert b"go/to" == actual_commonname2
+
+
+def test_numfile_sotkey():
+    assert utils.numfile_sortkey("00-xxx.sql") == (0, "-xxx.sql")
+    assert utils.numfile_sortkey("01-xxx.sql") == (1, "-xxx.sql")
+    assert utils.numfile_sortkey("10-xxx.sql") == (10, "-xxx.sql")
+    assert utils.numfile_sortkey("99-xxx.sql") == (99, "-xxx.sql")
+    assert utils.numfile_sortkey("100-xxx.sql") == (100, "-xxx.sql")
+    assert utils.numfile_sortkey("00100-xxx.sql") == (100, "-xxx.sql")
+    assert utils.numfile_sortkey("1.sql") == (1, ".sql")
+    assert utils.numfile_sortkey("1") == (1, "")
+    assert utils.numfile_sortkey("toto-01.sql") == (999999, "toto-01.sql")
diff --git a/swh/core/utils.py b/swh/core/utils.py
--- a/swh/core/utils.py
+++ b/swh/core/utils.py
@@ -8,6 +8,7 @@
 import itertools
 import os
 import re
+from typing import Tuple
 
 
 @contextmanager
@@ -109,14 +110,23 @@
     return path1.split(path0)[1]
 
 
-def numfile_sortkey(fname):
+def numfile_sortkey(fname: str) -> Tuple[int, str]:
     """Simple function to sort filenames of the form:
 
       nnxxx.ext
 
     where nn is a number according to the numbers.
 
+    Returns a tuple (order, remaining), where 'order' is the numeric (int)
+    value extracted from the file name, and 'remaining' is the remaining part
+    of the file name.
+
     Typically used to sort sql/nn-swh-xxx.sql files.
+
+    Unmatched file names will return 999999 as order value.
+
     """
-    num, rem = re.match(r"(\d*)(.*)", fname).groups()
-    return (num and int(num) or 99, rem)
+    m = re.match(r"(\d*)(.*)", fname)
+    assert m is not None
+    num, rem = m.groups()
+    return (int(num) if num else 999999, rem)