diff --git a/src/_zkapauthorizer/tests/test_storage_server.py b/src/_zkapauthorizer/tests/test_storage_server.py
index 84f852aa676efd27e08c7037572b25f1c2dba389..6dfbc491fe6132574b820ca4ee5d58a0fffb872d 100644
--- a/src/_zkapauthorizer/tests/test_storage_server.py
+++ b/src/_zkapauthorizer/tests/test_storage_server.py
@@ -649,6 +649,8 @@ class PassValidationTests(TestCase):
             Equals(1),
             "Unexpected histogram sum value",
         )
+        # If this bucket is 1 then all the other buckets must be 0, otherwise
+        # the sum above will be greater than 1.
         self.assertThat(
             after_bucket,
             Equals(1),