diff --git a/tox.ini b/tox.ini
index 600f8f6496..7a89e83fc9 100644
--- a/tox.ini
+++ b/tox.ini
@@ -73,8 +73,12 @@ commands = oslopolicy-sample-generator --config-file=etc/manila/manila-policy-ge
 commands = {posargs}
 
 [testenv:docs]
+# NOTE(elod.illes): requirements.txt is needed because otherwise
+# dependencies are installed during 'develop-inst' tox phase without
+# constraints which could cause failures in stable branches.
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/build