diff --git a/tests/ansible/integration/runner/custom_python_prehistoric_module.yml b/tests/ansible/integration/runner/custom_python_prehistoric_module.yml
index f2a3eefd..ebe34cc8 100644
--- a/tests/ansible/integration/runner/custom_python_prehistoric_module.yml
+++ b/tests/ansible/integration/runner/custom_python_prehistoric_module.yml
@@ -1,3 +1,7 @@
+# Test functionality of ansible_mitogen.runner.PREHISTORIC_HACK_RE, which
+# removes `reload(sys); sys.setdefaultencoding(...)` from an Ansible module
+# as it is sent to a target. There are probably very few modules in the wild
+# that still do this, if any - reload() is a Python 2.x builtin function.
 # issue #555
 
 - name: integration/runner/custom_python_prehistoric_module.yml
@@ -5,9 +9,11 @@
   tasks:
     - custom_python_prehistoric_module:
       register: out
+      when: is_mitogen
 
     - assert:
         that: out.ok
         fail_msg: out={{out}}
+      when: is_mitogen
   tags:
     - custom_python_prehistoric_module
diff --git a/tests/ansible/lib/modules/custom_python_prehistoric_module.py b/tests/ansible/lib/modules/custom_python_prehistoric_module.py
index 2268cbe6..61397488 100644
--- a/tests/ansible/lib/modules/custom_python_prehistoric_module.py
+++ b/tests/ansible/lib/modules/custom_python_prehistoric_module.py
@@ -1,5 +1,9 @@
 #!/usr/bin/python
 
+# Test functionality of ansible_mitogen.runner.PREHISTORIC_HACK_RE, which
+# removes `reload(sys); sys.setdefaultencoding(...)` from an Ansible module
+# as it is sent to a target. There are probably very few modules in the wild
+# that still do this, reload() is a Python 2.x builtin function.
 # issue #555: I'm a module that cutpastes an old hack.
 
 from ansible.module_utils.basic import AnsibleModule