diff --git a/.zuul.d/cross-jobs.yaml b/.zuul.d/cross-jobs.yaml
index 3ed1921a50..189889e662 100644
--- a/.zuul.d/cross-jobs.yaml
+++ b/.zuul.d/cross-jobs.yaml
@@ -14,6 +14,9 @@
          Use the specified tox environments (``ALL`` selects all).
     vars:
       zuul_work_dir: "{{ (zuul.projects | selectattr('required') | selectattr('name', 'match', '^(?!openstack/requirements)') | list)[0].src_dir }}"
+      # NOTE(jeblair): temporarily disabled tox_siblings until the fix
+      # in https://review.openstack.org/514058 lands.
+      tox_install_siblings: false
     files:
       - upper-constraints.txt
       - .zuul.d/cross-jobs.yaml
diff --git a/upper-constraints.txt b/upper-constraints.txt
index 56cad8bbab..9012ef73e6 100644
--- a/upper-constraints.txt
+++ b/upper-constraints.txt
@@ -278,7 +278,7 @@ stestr===1.1.0
 urwid===1.3.1;python_version=='3.4'
 urwid===1.3.1;python_version=='3.5'
 singledispatch===3.4.0.3;python_version=='2.7'
-oslo.serialization===2.21.2
+oslo.serialization===2.21.1
 warlock===1.2.0
 exabgp===4.0.2
 sphinxcontrib-httpdomain===1.5.0