diff --git a/tox.ini b/tox.ini
index b8ca21be..5ea6de46 100644
--- a/tox.ini
+++ b/tox.ini
@@ -7,7 +7,7 @@ stxdir = {toxinidir}/../
 [testenv]
 usedevelop = False
 install_command = pip install \
-    -chttps://opendev.org/openstack/requirements/raw/branch/stable/stein/upper-constraints.txt \
+    -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/stable/stein/upper-constraints.txt} \
     {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_CAPTURE=1