diff --git a/tox.ini b/tox.ini
index bdf16d9ce..110448956 100644
--- a/tox.ini
+++ b/tox.ini
@@ -5,6 +5,8 @@ envlist = lint,py,old_deps
 pass_env =
     SNAPSHOT_UPDATE
     POEDITOR_TOKEN
+# ensure that package deps match the explicit tox env deps:
+constrain_package_deps = true
 # use a constraints file for tox env and package deps:
 set_env =
     PIP_CONSTRAINT=constraints.txt