--- a/testing/mozharness/mozharness/base/python.py
+++ b/testing/mozharness/mozharness/base/python.py
@@ -461,16 +461,22 @@ class ResourceMonitoringMixin(object):
         super(ResourceMonitoringMixin, self).__init__(*args, **kwargs)
 
         self.register_virtualenv_module('psutil>=3.1.1', method='pip',
                                         optional=True)
         self.register_virtualenv_module('mozsystemmonitor==0.3',
                                         method='pip', optional=True)
         self.register_virtualenv_module('jsonschema==2.5.1',
                                         method='pip')
+        # explicitly install functools32, because some slaves aren't using
+        # a version of pip recent enough to install it automatically with
+        # jsonschema (which depends on it)
+        # https://github.com/Julian/jsonschema/issues/233
+        self.register_virtualenv_module('functools32==3.2.3-2',
+                                        method='pip')
         self._resource_monitor = None
 
         # 2-tuple of (name, options) to assign Perfherder resource monitor
         # metrics to. This needs to be assigned by a script in order for
         # Perfherder metrics to be reported.
         self.resource_monitor_perfherder_id = None
 
     @PostScriptAction('create-virtualenv')