diff --git a/storage/sealer/mock/mock.go b/storage/sealer/mock/mock.go
index baa644cc1..5b0afd35c 100644
--- a/storage/sealer/mock/mock.go
+++ b/storage/sealer/mock/mock.go
@@ -439,6 +439,9 @@ func (mgr *SectorMgr) GenerateWindowPoStWithVanilla(ctx context.Context, proofTy
 func (mgr *SectorMgr) ReadPiece(ctx context.Context, sector storiface.SectorRef, offset storiface.UnpaddedByteIndex, size abi.UnpaddedPieceSize, ticket abi.SealRandomness, unsealed cid.Cid) (mount.Reader, bool, error) {
 	off := storiface.UnpaddedByteIndex(0)
 	var piece cid.Cid
+
+	mgr.lk.Lock()
+
 	for _, c := range mgr.sectors[sector.ID].pieces {
 		piece = c
 		if off >= offset {
@@ -451,6 +454,8 @@ func (mgr *SectorMgr) ReadPiece(ctx context.Context, sector storiface.SectorRef,
 	}
 	br := bytes.NewReader(mgr.pieces[piece][:size])
 
+	mgr.lk.Unlock()
+
 	return struct {
 		io.ReadCloser
 		io.Seeker