diff --git a/tools/check_main_sync.py b/tools/check_main_sync.py
index fa3265d310..12d5b78a32 100644
--- a/tools/check_main_sync.py
+++ b/tools/check_main_sync.py
@@ -97,12 +97,17 @@ if __name__ == "__main__":
     parser.add_argument("--strict", action="store_true", help="also fail when the branch is behind the base")
     args = parser.parse_args()
     try:
-        base = resolve_base(args.repo, args.base)
-        if args.fetch:
-            remote, _, branch = base.partition("/")
+        def fetch(ref):
+            remote, _, branch = ref.partition("/")
             if not branch:
                 parser.error("--fetch needs a remote-tracking base such as upstream/main")
             git(args.repo, "fetch", "--quiet", remote, branch)
+
+        if args.fetch and args.base:
+            fetch(args.base)
+        base = resolve_base(args.repo, args.base)
+        if args.fetch and not args.base:
+            fetch(base)
         data = report(args.repo, base, args.head)
     except RuntimeError as error:
         parser.error(str(error))
diff --git a/tools/tests/test_check_main_sync.py b/tools/tests/test_check_main_sync.py
index 4a13a56e0a..d893e70b0e 100644
--- a/tools/tests/test_check_main_sync.py
+++ b/tools/tests/test_check_main_sync.py
@@ -177,6 +177,14 @@ class CheckMainSyncTests(unittest.TestCase):
         self.assertEqual(result.returncode, 2)
         self.assertIn("no base ref found", result.stderr)
 
+    def test_explicit_base_is_fetched_before_it_is_resolved(self):
+        with tempfile.TemporaryDirectory() as other:
+            subprocess.run(["git", "clone", "-q", self.directory.name, other], check=True, capture_output=True)
+            self.repo.git("remote", "add", "upstream", other)
+            result = subprocess.run([sys.executable, str(TOOL), "--repo", self.directory.name, "--base", "upstream/main", "--fetch"], capture_output=True, text=True)
+            self.assertEqual(result.returncode, 0, result.stderr)
+            self.assertIn("merge: clean", result.stdout)
+
 
 if __name__ == "__main__":
     unittest.main()