diff --git a/src/_zkapauthorizer/storage_common.py b/src/_zkapauthorizer/storage_common.py
index a41f40a89a59c5212b8a7b8187739577dc12454a..d5a24ff7ec0deb9241f8ce4b99dc8ed185fcff46 100644
--- a/src/_zkapauthorizer/storage_common.py
+++ b/src/_zkapauthorizer/storage_common.py
@@ -118,6 +118,9 @@ def get_implied_data_length(data_vector, new_length):
     """
     :param data_vector: See ``allmydata.interfaces.DataVector``.
 
+    :param new_length: See
+        ``allmydata.interfaces.RIStorageServer.slot_testv_and_readv_and_writev``.
+
     :return int: The amount of data, in bytes, implied by a data vector and a
         size.
     """
diff --git a/src/_zkapauthorizer/tests/test_storage_server.py b/src/_zkapauthorizer/tests/test_storage_server.py
index 7f3c22864425727ef5e4ccced3ce3a51a3b0df4a..07cc6e16ccf007061306d8eb4f8ff62a86e4588f 100644
--- a/src/_zkapauthorizer/tests/test_storage_server.py
+++ b/src/_zkapauthorizer/tests/test_storage_server.py
@@ -429,8 +429,8 @@ class PassValidationTests(TestCase):
     )
     def test_immutable_share_sizes(self, storage_index, secrets, sharenums, allocated_size):
         """
-        ``share_sizes`` returns the size of the requested shares in the requested
-        storage_index
+        ``share_sizes`` returns the size of the requested iimutable shares in the
+        requested storage index.
         """
         # hypothesis causes our storage server to be used many times.  Clean
         # up between iterations.
@@ -474,6 +474,10 @@ class PassValidationTests(TestCase):
         test_and_write_vectors_for_shares=test_and_write_vectors_for_shares(),
     )
     def test_mutable_share_sizes(self, slot, secrets, sharenums, test_and_write_vectors_for_shares):
+        """
+        ``share_sizes`` returns the size of the requested mutable shares in the
+        requested slot.
+        """
         # hypothesis causes our storage server to be used many times.  Clean
         # up between iterations.
         cleanup_storage_server(self.anonymous_storage_server)