diff --git a/swh/loader/mercurial/loader.py b/swh/loader/mercurial/loader.py
--- a/swh/loader/mercurial/loader.py
+++ b/swh/loader/mercurial/loader.py
@@ -278,10 +278,10 @@
             except CommandErrorWrapper as e:
                 for msg in [
                     b"does not appear to be an hg repository",
-                    b"404: Not Found",
-                    b"Name or service not known",
+                    b"404: not found",
+                    b"name or service not known",
                 ]:
-                    if msg in e.err:
+                    if msg in e.err.lower():
                         raise NotFound(e.args[0]) from None
                 raise e
 
diff --git a/swh/loader/mercurial/tests/test_loader.py b/swh/loader/mercurial/tests/test_loader.py
--- a/swh/loader/mercurial/tests/test_loader.py
+++ b/swh/loader/mercurial/tests/test_loader.py
@@ -235,9 +235,10 @@
 @pytest.mark.parametrize(
     "error_msg",
     [
-        b"does not appear to be an hg repository",
+        b"does not appear to be an HG repository",
         b"404: Not Found",
-        b" Name or service not known",
+        b"404: NOT FOUND",
+        b"Name or service not known",
     ],
 )
 def test_visit_error_with_status_not_found(