torrent.go | 3 ++-

diff --git a/torrent.go b/torrent.go
index f7f518f8b75f261c34dee1e857db0857c45cfcd4..0f4a681024302451c2e7b247ea49324eb014f9f8 100644
--- a/torrent.go
+++ b/torrent.go
@@ -769,7 +769,8 @@ 	p := t.piece(piece)
 	p.waitNoPendingWrites()
 	ip := t.info.Piece(int(piece))
 	pl := ip.Length()
-	_, copyErr = io.Copy(hash, io.NewSectionReader(t.pieces[piece].Storage(), 0, pl))
+	_, copyErr = io.CopyN( // Return no error iff pl bytes are copied.
+		hash, io.NewSectionReader(t.pieces[piece].Storage(), 0, pl), pl)
 	missinggo.CopyExact(&ret, hash.Sum(nil))
 	return
 }