file.go    | 12 +++++++++---
 torrent.go | 17 ++++++++++++-----

diff --git a/file.go b/file.go
index 7cc0ad64bf5d67f520ba37863f8c7d368fb78b13..5983b855e1d46f470b8b804aac05a1dea333c423 100644
--- a/file.go
+++ b/file.go
@@ -121,7 +121,7 @@ 	if prio == f.prio {
 		return
 	}
 	f.prio = prio
-	f.t.updatePiecePriorities(f.firstPieceIndex().Int(), f.lastPieceIndex().Int()+1)
+	f.t.updatePiecePriorities(f.firstPieceIndex().Int(), f.endPieceIndex().Int())
 }
 
 // Returns the priority per File.SetPriority.
@@ -132,9 +132,15 @@ 	return f.prio
 }
 
 func (f *File) firstPieceIndex() pwp.Integer {
+	if f.t.usualPieceSize() == 0 {
+		return 0
+	}
 	return pwp.Integer(f.offset / int64(f.t.usualPieceSize()))
 }
 
-func (f *File) lastPieceIndex() pwp.Integer {
-	return pwp.Integer((f.offset + f.length) / int64(f.t.usualPieceSize()))
+func (f *File) endPieceIndex() pwp.Integer {
+	if f.t.usualPieceSize() == 0 {
+		return 0
+	}
+	return pwp.Integer((f.offset+f.length-1)/int64(f.t.usualPieceSize())) + 1
 }
diff --git a/torrent.go b/torrent.go
index 981f5b85a83776e7eeb9bd8fb55fed93b27985e1..6440a3f823636e4925d6465ad82c6be5a56985f7 100644
--- a/torrent.go
+++ b/torrent.go
@@ -305,26 +305,33 @@ 		piece.t = t
 		piece.index = i
 		piece.noPendingWrites.L = &piece.pendingWritesMutex
 		missinggo.CopyExact(piece.hash[:], hash)
-		piece.files = (*t.files)[pieceFirstFileIndex(piece.torrentBeginOffset(), *t.files):pieceLastFileIndex(piece.torrentEndOffset(), *t.files)]
+		files := *t.files
+		beginFile := pieceFirstFileIndex(piece.torrentBeginOffset(), files)
+		endFile := pieceEndFileIndex(piece.torrentEndOffset(), files)
+		piece.files = files[beginFile:endFile]
 	}
 }
 
+// Returns the index of the first file containing the piece. files must be
+// ordered by offset.
 func pieceFirstFileIndex(pieceOffset int64, files []*File) int {
 	for i, f := range files {
 		if f.offset+f.length > pieceOffset {
 			return i
 		}
 	}
-	return -1
+	return 0
 }
 
-func pieceLastFileIndex(pieceEndOffset int64, files []*File) int {
+// Returns the index after the last file containing the piece. files must be
+// ordered by offset.
+func pieceEndFileIndex(pieceEndOffset int64, files []*File) int {
 	for i, f := range files {
 		if f.offset+f.length >= pieceEndOffset {
-			return i
+			return i + 1
 		}
 	}
-	return -1
+	return 0
 }
 
 func (t *Torrent) cacheLength(info *metainfo.Info) {