diff --git a/chain/sync.go b/chain/sync.go
index 1b9a302f7..4dccc2036 100644
--- a/chain/sync.go
+++ b/chain/sync.go
@@ -886,6 +886,35 @@ func (syncer *Syncer) syncFork(ctx context.Context, incoming *types.TipSet, know
 		}
 	}
 
+	incomingParentsTsk := incoming.Parents()
+	commonParent := false
+	for _, incomingParent := range incomingParentsTsk.Cids() {
+		if known.Contains(incomingParent) {
+			commonParent = true
+		}
+	}
+
+	if commonParent {
+		// known contains at least one of incoming's Parents => the common ancestor is known's Parents (incoming's Grandparents)
+		// in this case, we need to return {incoming.Parents()}
+		incomingParents, err := syncer.store.LoadTipSet(ctx, incomingParentsTsk)
+		if err != nil {
+			// fallback onto the network
+			tips, err := syncer.Exchange.GetBlocks(ctx, incoming.Parents(), 1)
+			if err != nil {
+				return nil, xerrors.Errorf("failed to fetch incomingParents from the network: %w", err)
+			}
+
+			if len(tips) == 0 {
+				return nil, xerrors.Errorf("network didn't return any tipsets")
+			}
+
+			incomingParents = tips[0]
+		}
+
+		return []*types.TipSet{incomingParents}, nil
+	}
+
 	// TODO: Does this mean we always ask for ForkLengthThreshold blocks from the network, even if we just need, like, 2? Yes.
 	// Would it not be better to ask in smaller chunks, given that an ~ForkLengthThreshold is very rare?
 	tips, err := syncer.Exchange.GetBlocks(ctx, incoming.Parents(), int(build.ForkLengthThreshold))
diff --git a/storage/sealer/worker_local.go b/storage/sealer/worker_local.go
index 9f2f2efdc..7fc494955 100644
--- a/storage/sealer/worker_local.go
+++ b/storage/sealer/worker_local.go
@@ -664,7 +664,7 @@ func (l *LocalWorker) GenerateWindowPoStAdv(ctx context.Context, ppt abi.Registe
 	var wg sync.WaitGroup
 	wg.Add(len(sectors))
 
-	vproofs := make([][]byte, 0, len(sectors))
+	vproofs := make([][]byte, len(sectors))
 
 	for i, s := range sectors {
 		if l.challengeThrottle != nil {
@@ -702,8 +702,7 @@ func (l *LocalWorker) GenerateWindowPoStAdv(ctx context.Context, ppt abi.Registe
 				return
 			}
 
-			//vproofs[i] = vanilla // todo substitutes??
-			vproofs = append(vproofs, vanilla)
+			vproofs[i] = vanilla
 		}(i, s)
 	}
 	wg.Wait()
@@ -717,6 +716,22 @@ func (l *LocalWorker) GenerateWindowPoStAdv(ctx context.Context, ppt abi.Registe
 		return storiface.WindowPoStResult{Skipped: skipped}, nil
 	}
 
+	// compact skipped sectors
+	var skippedSoFar int
+	for i := range vproofs {
+		if len(vproofs[i]) == 0 {
+			skippedSoFar++
+			continue
+		}
+
+		if skippedSoFar > 0 {
+			vproofs[i-skippedSoFar] = vproofs[i]
+		}
+	}
+
+	vproofs = vproofs[:len(vproofs)-skippedSoFar]
+
+	// compute the PoSt!
 	res, err := sb.GenerateWindowPoStWithVanilla(ctx, ppt, mid, randomness, vproofs, partitionIdx)
 	r := storiface.WindowPoStResult{
 		PoStProofs: res,