storage/completion.go           | 2 +-
 storage/completion_piece_map.go | 9 ++++++++-

diff --git a/storage/completion.go b/storage/completion.go
index 15fcc249d9a3ebfbf201094866a599c349f779fb..7804351036a238c25b9a3b0ddf06ae312d492dcc 100644
--- a/storage/completion.go
+++ b/storage/completion.go
@@ -6,7 +6,7 @@
 	"github.com/anacrolix/torrent/metainfo"
 )
 
-// Implementations track the completion of pieces.
+// Implementations track the completion of pieces. It must be concurrent-safe.
 type pieceCompletion interface {
 	Get(metainfo.PieceKey) (bool, error)
 	Set(metainfo.PieceKey, bool) error
diff --git a/storage/completion_piece_map.go b/storage/completion_piece_map.go
index cca1324510ba79d6adff99895ac3e3f1c45a5d63..f4b4e28582d0744b7e46e2c913436c1d323b8187 100644
--- a/storage/completion_piece_map.go
+++ b/storage/completion_piece_map.go
@@ -1,21 +1,27 @@
 package storage
 
 import (
+	"sync"
+
 	"github.com/anacrolix/torrent/metainfo"
 )
 
 type mapPieceCompletion struct {
-	m map[metainfo.PieceKey]struct{}
+	mu sync.Mutex
+	m  map[metainfo.PieceKey]struct{}
 }
 
 func (mapPieceCompletion) Close() error { return nil }
 
 func (me *mapPieceCompletion) Get(pk metainfo.PieceKey) (bool, error) {
+	me.mu.Lock()
 	_, ok := me.m[pk]
+	me.mu.Unlock()
 	return ok, nil
 }
 
 func (me *mapPieceCompletion) Set(pk metainfo.PieceKey, b bool) error {
+	me.mu.Lock()
 	if b {
 		if me.m == nil {
 			me.m = make(map[metainfo.PieceKey]struct{})
@@ -24,5 +30,6 @@ 		me.m[pk] = struct{}{}
 	} else {
 		delete(me.m, pk)
 	}
+	me.mu.Unlock()
 	return nil
 }