diff --git a/tox.ini b/tox.ini
index ab4397a237f7..85d7a54c620b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -43,6 +43,9 @@ commands =
   bash tools/flake8wrap.sh {posargs}
 
 [testenv:py34]
+# NOTE(mriedem): If py34 fails with "db type could not be determined", delete
+# .testrepository and try again. Running py34 before py27 is OK, but not the
+# other way around. See: https://bugs.launchpad.net/testrepository/+bug/1212909
 setenv = {[testenv]setenv}
 commands =
   find . -type f -name "*.pyc" -delete