diff --git a/selftests/functional/test_basic.py b/selftests/functional/test_basic.py
index e4b880899a56e11fcdae0642c7e51ec6639d57d0..7bf2d3591b71848d8706a1bf91a621cb56da7ca6 100644
--- a/selftests/functional/test_basic.py
+++ b/selftests/functional/test_basic.py
@@ -186,7 +186,7 @@ class RunnerOperationTest(unittest.TestCase):
                 os.mkdir(value)
             config += "%s = %s\n" % (key, value)
         fd, config_file = tempfile.mkstemp(dir=self.tmpdir)
-        os.write(fd, config)
+        os.write(fd, config.encode())
         os.close(fd)
 
         cmd = '%s --config %s config --datadir' % (AVOCADO, config_file)