diff --git a/swh/core/tests/test_config.py b/swh/core/tests/test_config.py
index 81c295c..0b0bb73 100644
--- a/swh/core/tests/test_config.py
+++ b/swh/core/tests/test_config.py
@@ -1,78 +1,78 @@
 # Copyright (C) 2015  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 import tempfile
 import unittest
 import os
 import shutil
 
 from nose.tools import istest
 
 from swh.core import config
 
 
 class ConfReaderTest(unittest.TestCase):
 
     @classmethod
     def setUp(self):
         # create a temporary folder
         self.tmpdir = tempfile.mkdtemp(prefix='test-swh-core.')
         self.conffile = os.path.join(self.tmpdir, 'config.ini')
         with open(self.conffile, 'w') as conf:
             conf.write("""[main]
 a = 1
 b = this is a string
 c = true
 """)
 
     @classmethod
     def tearDown(self):
         shutil.rmtree(self.tmpdir)
 
     @istest
     def read(self):
         # given
         default_conf = {'a': ('int', 2),
                         'b': ('string', 'default-string'),
                         'c': ('bool', True),
                         'd': ('int', 10)}
 
         # when
         res = config.read(self.conffile, default_conf)
 
         # then
         self.assertEquals(res, {'a': 1,
                                 'b': 'this is a string',
                                 'c': True,
                                 'd': 10})
 
     @istest
     def prepare_folder(self):
         # given
-        conf = {'path1': self.tmpdir + 'path1',
-                'path2': self.tmpdir + 'path2/depth1'}
+        conf = {'path1': os.path.join(self.tmpdir, 'path1'),
+                'path2': os.path.join(self.tmpdir, 'path2', 'depth1')}
 
         # the folders does not exists
         self.assertFalse(os.path.exists(conf['path1']),
                          "path1 should not exist.")
         self.assertFalse(os.path.exists(conf['path2']),
                          "path2 should not exist.")
 
         # when
         config.prepare_folders(conf, 'path1')
 
         # path1 exists but not path2
         self.assertTrue(os.path.exists(conf['path1']),
                         "path1 should now exist!")
         self.assertFalse(os.path.exists(conf['path2']),
                          "path2 should not exist.")
 
         # path1 already exists, skips it but creates path2
         config.prepare_folders(conf, 'path1', 'path2')
 
         self.assertTrue(os.path.exists(conf['path1']),
                         "path1 should still exist!")
         self.assertTrue(os.path.exists(conf['path2']),
                         "path2 should now exist.")