diff --git a/zaza/charm_tests/security/tests.py b/zaza/charm_tests/security/tests.py
index a174c60..fd6db82 100644
--- a/zaza/charm_tests/security/tests.py
+++ b/zaza/charm_tests/security/tests.py
@@ -26,7 +26,7 @@ from zaza.utilities.file_assertions import (
 )
 
 
-def _make_test_function(application, file_details, paths=[]):
+def _make_test_function(application, file_details, paths=None):
     def test(self):
         for unit in model.get_units(application):
             unit = unit.entity_id
diff --git a/zaza/utilities/file_assertions.py b/zaza/utilities/file_assertions.py
index 470fcd2..5993f81 100644
--- a/zaza/utilities/file_assertions.py
+++ b/zaza/utilities/file_assertions.py
@@ -17,8 +17,10 @@
 import zaza.model as model
 
 
-def assert_path_glob(test_case, unit, file_details, paths=[]):
+def assert_path_glob(test_case, unit, file_details, paths=None):
     """Verify all files in a given directory."""
+    if not paths:
+        paths = []
     result = model.run_on_unit(
         unit, 'bash -c "'
         'shopt -s -q globstar; '