diff --git a/swh/storage/api/server.py b/swh/storage/api/server.py
--- a/swh/storage/api/server.py
+++ b/swh/storage/api/server.py
@@ -101,7 +101,7 @@
 
     cfg = config.read(config_path)
     if "storage" not in cfg:
-        raise KeyError("Missing '%storage' configuration")
+        raise KeyError("Missing 'storage' configuration")
 
     return cfg
 
diff --git a/swh/storage/tests/test_server.py b/swh/storage/tests/test_server.py
--- a/swh/storage/tests/test_server.py
+++ b/swh/storage/tests/test_server.py
@@ -48,7 +48,7 @@
 def test_load_and_check_config_wrong_configuration(tmpdir):
     """Wrong configuration raises"""
     config_path = prepare_config_file(tmpdir, "something: useless")
-    with pytest.raises(KeyError, match="Missing '%storage' configuration"):
+    with pytest.raises(KeyError, match="Missing 'storage' configuration"):
         load_and_check_config(config_path)