diff --git a/src/Tahoe/Download.hs b/src/Tahoe/Download.hs
index 55d94812799652b049f842e20ddb2ec960849430..c1364336a673e45c1ddcee8001b2bc5950884d9f 100644
--- a/src/Tahoe/Download.hs
+++ b/src/Tahoe/Download.hs
@@ -34,7 +34,9 @@ data DownloadError
         { notEnoughDecodedSharesNeeded :: Int
         , notEnoughDecodedSharesFound :: Int
         }
-    | ShareDecodingFailed
+    | -- | Enough syntactically valid shares were recovered but they could not
+      -- be interpreted.
+      ShareDecodingFailed
     | -- | An attempt was made to download a share but no servers were given for
       -- the download.
       NoServers