diff --git a/.github/workflows/tox.yaml b/.github/workflows/tox.yaml
index ddfeff1e0..16fffe08a 100644
--- a/.github/workflows/tox.yaml
+++ b/.github/workflows/tox.yaml
@@ -20,7 +20,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install tox tox-gh-actions
+        pip install "tox<4.0" tox-gh-actions
     - name: Lint with tox
       run: tox -e pep8
     - name: Test with tox
diff --git a/requirements.txt b/requirements.txt
index a6a567e42..f42b109cd 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -13,7 +13,8 @@ juju_wait
 # https://github.com/pyca/pyopenssl/commit/a145fc3bc6d2e943434beb2f04bbf9b18930296f
 pyopenssl<22.1.0
 
-PyYAML<=4.2,>=3.0
+PyYAML<=4.2,>=3.0; python_version < '3.9'
+PyYAML>4.2; python_version >='3.9'
 flake8>=2.2.4
 flake8-docstrings
 flake8-per-file-ignores
diff --git a/setup.py b/setup.py
index 8fe59bfae..5f4f4f963 100644
--- a/setup.py
+++ b/setup.py
@@ -69,7 +69,7 @@
 ]
 
 tests_require = [
-    'tox >= 2.3.1',
+    'tox',
 ]
 
 
diff --git a/tox.ini b/tox.ini
index 2a8ae4b34..68e149122 100644
--- a/tox.ini
+++ b/tox.ini
@@ -15,12 +15,16 @@ skip_missing_interpreters = False
 #   https://stackoverflow.com/a/38133283
 requires = pip < 20.3
            virtualenv < 20.0
+           tox < 4.0.0
 # NOTE: https://wiki.canonical.com/engineering/OpenStack/InstallLatestToxOnOsci
 minversion = 3.2.0
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
+allowlist_externals =
+    {toxinidir}/pip.sh
+
 install_command =
   {toxinidir}/pip.sh install {opts} {packages}
 
diff --git a/unit_tests/utilities/test_zaza_utilities_series_upgrade.py b/unit_tests/utilities/test_zaza_utilities_series_upgrade.py
index 340fc596e..cf0ab8e60 100644
--- a/unit_tests/utilities/test_zaza_utilities_series_upgrade.py
+++ b/unit_tests/utilities/test_zaza_utilities_series_upgrade.py
@@ -77,7 +77,7 @@ def test_series_upgrade(self):
             _unit, _machine_num, origin=_origin,
             to_series=_to_series, from_series=_from_series,
             workaround_script=_workaround_script, files=_files)
-        self.block_until_all_units_idle.called_with()
+        self.block_until_all_units_idle.assert_called()
         self.prepare_series_upgrade.assert_called_once_with(
             _machine_num, to_series=_to_series)
         self.wrap_do_release_upgrade.assert_called_once_with(
diff --git a/zaza/openstack/charm_tests/rabbitmq_server/tests.py b/zaza/openstack/charm_tests/rabbitmq_server/tests.py
index 027ef957d..9b18d8dca 100644
--- a/zaza/openstack/charm_tests/rabbitmq_server/tests.py
+++ b/zaza/openstack/charm_tests/rabbitmq_server/tests.py
@@ -271,6 +271,13 @@ def _retry_check_commands_on_units(self, cmds, units):
 
     def test_414_rmq_nrpe_monitors(self):
         """Check rabbimq-server nrpe monitor basic functionality."""
+        try:
+            zaza.model.get_application("nrpe")
+        except KeyError:
+            logging.warn(("Skipping as nrpe is not deployed. "
+                          "http://pad.lv/1968008"))
+            return
+
         units = zaza.model.get_units(self.application_name)
         host_names = generic_utils.get_unit_hostnames(units)