diff --git a/src/_zkapauthorizer/tests/storage_common.py b/src/_zkapauthorizer/tests/storage_common.py
index e0e2c1a2b2d030c87b058e1ba843ea6947497634..5141dd58cd64ea5bf126a1364e8e2d13f8f942dd 100644
--- a/src/_zkapauthorizer/tests/storage_common.py
+++ b/src/_zkapauthorizer/tests/storage_common.py
@@ -272,6 +272,9 @@ class _PassFactory(object):
         return PassGroup(message, self, zip(passes, passes))
 
     def _clear(self):
+        """
+        Forget about all passes: returned, in use, spent, invalid, issued.
+        """
         del self.returned[:]
         self.in_use.clear()
         self.invalid.clear()