diff --git a/extern/sector-storage/mock/mock.go b/extern/sector-storage/mock/mock.go
index 3acf3dc41..eeb1404ad 100644
--- a/extern/sector-storage/mock/mock.go
+++ b/extern/sector-storage/mock/mock.go
@@ -385,11 +385,11 @@ func generateFakePoSt(sectorInfo []proof5.SectorInfo, rpt func(abi.RegisteredSea
 }
 
 func (mgr *SectorMgr) ReadPiece(ctx context.Context, sector storage.SectorRef, offset storiface.UnpaddedByteIndex, startOffset uint64, size abi.UnpaddedPieceSize, ticket abi.SealRandomness, unsealed cid.Cid) (io.ReadCloser, bool, error) {
-	if uint64(offset)+startOffset != 0 {
+	if uint64(offset) != 0 {
 		panic("implme")
 	}
 
-	return ioutil.NopCloser(bytes.NewReader(mgr.pieces[mgr.sectors[sector.ID].pieces[0]][:size])), false, nil
+	return ioutil.NopCloser(bytes.NewReader(mgr.pieces[mgr.sectors[sector.ID].pieces[0]][startOffset:size])), false, nil
 }
 
 func (mgr *SectorMgr) StageFakeData(mid abi.ActorID, spt abi.RegisteredSealProof) (storage.SectorRef, []abi.PieceInfo, error) {