diff --git a/chain/actors/actors_test.go b/chain/actors/actors_test.go
index 6611e8c51..5a3be6c13 100644
--- a/chain/actors/actors_test.go
+++ b/chain/actors/actors_test.go
@@ -51,7 +51,8 @@ func setupVMTestEnv(t *testing.T) (*vm.VM, []address.Address) {
 
 	cs := store.NewChainStore(bs, nil)
 
-	vm, err := vm.NewVM(stateroot, 1, maddr, cs)
+	// TODO: should probabaly mock out the randomness bit, nil works for now
+	vm, err := vm.NewVM(stateroot, 1, nil, maddr, cs)
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/chain/actors/harness2_test.go b/chain/actors/harness2_test.go
index 55d766a8c..3e08a2bc2 100644
--- a/chain/actors/harness2_test.go
+++ b/chain/actors/harness2_test.go
@@ -158,7 +158,7 @@ func NewHarness(t *testing.T, options ...HarnessOpt) *Harness {
 		t.Fatal(err)
 	}
 	h.cs = store.NewChainStore(h.bs, nil)
-	h.vm, err = vm.NewVM(stateroot, 1, h.HI.Miner, h.cs)
+	h.vm, err = vm.NewVM(stateroot, 1, nil, h.HI.Miner, h.cs)
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/chain/gen/gen.go b/chain/gen/gen.go
index 9a06af5af..e4b700196 100644
--- a/chain/gen/gen.go
+++ b/chain/gen/gen.go
@@ -360,7 +360,7 @@ type mca struct {
 }
 
 func (mca mca) ChainGetRandomness(ctx context.Context, pts *types.TipSet, ticks []*types.Ticket, lb int) ([]byte, error) {
-	return mca.sm.ChainStore().GetRandomness(ctx, pts, ticks, lb)
+	return mca.sm.ChainStore().GetRandomness(ctx, pts, ticks, int64(lb))
 }
 
 func (mca mca) StateMinerPower(ctx context.Context, maddr address.Address, ts *types.TipSet) (api.MinerPower, error) {
diff --git a/chain/gen/mining.go b/chain/gen/mining.go
index 0d484f4ca..3b34e5d56 100644
--- a/chain/gen/mining.go
+++ b/chain/gen/mining.go
@@ -27,7 +27,8 @@ func MinerCreateBlock(ctx context.Context, sm *stmgr.StateManager, w *wallet.Wal
 
 	height := parents.Height() + uint64(len(tickets))
 
-	vmi, err := vm.NewVM(st, height, miner, sm.ChainStore())
+	r := vm.NewChainRand(sm.ChainStore(), parents, tickets)
+	vmi, err := vm.NewVM(st, height, r, miner, sm.ChainStore())
 	if err != nil {
 		return nil, err
 	}
diff --git a/chain/gen/utils.go b/chain/gen/utils.go
index 7ee1053de..1a1f9fcb4 100644
--- a/chain/gen/utils.go
+++ b/chain/gen/utils.go
@@ -169,7 +169,7 @@ func mustEnc(i cbg.CBORMarshaler) []byte {
 }
 
 func SetupStorageMiners(ctx context.Context, cs *store.ChainStore, sroot cid.Cid, gmcfg *GenMinerCfg) (cid.Cid, error) {
-	vm, err := vm.NewVM(sroot, 0, actors.NetworkAddress, cs)
+	vm, err := vm.NewVM(sroot, 0, nil, actors.NetworkAddress, cs)
 	if err != nil {
 		return cid.Undef, xerrors.Errorf("failed to create NewVM: %w", err)
 	}
diff --git a/chain/stmgr/call.go b/chain/stmgr/call.go
index 607273724..3c5af3079 100644
--- a/chain/stmgr/call.go
+++ b/chain/stmgr/call.go
@@ -11,8 +11,8 @@ import (
 	"golang.org/x/xerrors"
 )
 
-func (sm *StateManager) CallRaw(ctx context.Context, msg *types.Message, bstate cid.Cid, bheight uint64) (*types.MessageReceipt, error) {
-	vmi, err := vm.NewVM(bstate, bheight, actors.NetworkAddress, sm.cs)
+func (sm *StateManager) CallRaw(ctx context.Context, msg *types.Message, bstate cid.Cid, r vm.Rand, bheight uint64) (*types.MessageReceipt, error) {
+	vmi, err := vm.NewVM(bstate, bheight, r, actors.NetworkAddress, sm.cs)
 	if err != nil {
 		return nil, xerrors.Errorf("failed to set up vm: %w", err)
 	}
@@ -57,5 +57,7 @@ func (sm *StateManager) Call(ctx context.Context, msg *types.Message, ts *types.
 		return nil, err
 	}
 
-	return sm.CallRaw(ctx, msg, state, ts.Height())
+	r := vm.NewChainRand(sm.cs, ts, nil)
+
+	return sm.CallRaw(ctx, msg, state, r, ts.Height())
 }
diff --git a/chain/stmgr/stmgr.go b/chain/stmgr/stmgr.go
index 03f14a371..e72e53d3f 100644
--- a/chain/stmgr/stmgr.go
+++ b/chain/stmgr/stmgr.go
@@ -78,7 +78,9 @@ func (sm *StateManager) computeTipSetState(cids []cid.Cid) (cid.Cid, error) {
 		return cid.Undef, xerrors.Errorf("recursive TipSetState failed: %w", err)
 	}
 
-	vmi, err := vm.NewVM(pstate, ts.Height(), address.Undef, sm.cs)
+	r := vm.NewChainRand(sm.cs, ts, nil)
+
+	vmi, err := vm.NewVM(pstate, ts.Height(), r, address.Undef, sm.cs)
 	if err != nil {
 		return cid.Undef, xerrors.Errorf("instantiating VM failed: %w", err)
 	}
diff --git a/chain/stmgr/utils.go b/chain/stmgr/utils.go
index a9707acc9..07e6115ec 100644
--- a/chain/stmgr/utils.go
+++ b/chain/stmgr/utils.go
@@ -22,7 +22,7 @@ func GetMinerWorker(ctx context.Context, sm *StateManager, st cid.Cid, maddr add
 		To:     maddr,
 		From:   maddr,
 		Method: actors.MAMethods.GetWorkerAddr,
-	}, st, 0)
+	}, st, nil, 0)
 	if err != nil {
 		return address.Undef, xerrors.Errorf("callRaw failed: %w", err)
 	}
@@ -48,7 +48,7 @@ func GetMinerOwner(ctx context.Context, sm *StateManager, st cid.Cid, maddr addr
 		To:     maddr,
 		From:   maddr,
 		Method: actors.MAMethods.GetOwner,
-	}, st, 0)
+	}, st, nil, 0)
 	if err != nil {
 		return address.Undef, xerrors.Errorf("callRaw failed: %w", err)
 	}
diff --git a/chain/store/store.go b/chain/store/store.go
index 204b7de98..2298db7ef 100644
--- a/chain/store/store.go
+++ b/chain/store/store.go
@@ -701,16 +701,20 @@ func (cs *ChainStore) TryFillTipSet(ts *types.TipSet) (*FullTipSet, error) {
 	return NewFullTipSet(out), nil
 }
 
-func (cs *ChainStore) GetRandomness(ctx context.Context, pts *types.TipSet, tickets []*types.Ticket, lb int) ([]byte, error) {
-	if lb < len(tickets) {
+func (cs *ChainStore) GetRandomness(ctx context.Context, pts *types.TipSet, tickets []*types.Ticket, lb int64) ([]byte, error) {
+	if lb < 0 {
+		return nil, fmt.Errorf("negative lookback parameters are not valid (got %d)", lb)
+	}
+	lt := int64(len(tickets))
+	if lb < lt {
 		log.Warn("self sampling randomness. this should be extremely rare, if you see this often it may be a bug")
 
-		t := tickets[len(tickets)-(1+lb)]
+		t := tickets[lt-(1+lb)]
 
 		return t.VDFResult, nil
 	}
 
-	nv := lb - len(tickets)
+	nv := lb - lt
 
 	nextCids := pts.Cids()
 	for {
@@ -720,13 +724,14 @@ func (cs *ChainStore) GetRandomness(ctx context.Context, pts *types.TipSet, tick
 		}
 
 		mtb := nts.MinTicketBlock()
-		if nv < len(mtb.Tickets) {
-			t := mtb.Tickets[len(mtb.Tickets)-(1+nv)]
-			log.Infof("Returning randomness: H:%d, t:%d, mtb:%s", nts.Height(), len(mtb.Tickets)-(1+nv), mtb.Cid())
+		lt := int64(len(mtb.Tickets))
+		if nv < lt {
+			t := mtb.Tickets[lt-(1+nv)]
+			log.Infof("Returning randomness: H:%d, t:%d, mtb:%s", nts.Height(), lt-(1+nv), mtb.Cid())
 			return t.VDFResult, nil
 		}
 
-		nv -= len(mtb.Tickets)
+		nv -= lt
 
 		// special case for lookback behind genesis block
 		// TODO(spec): this is not in the spec, need to sync that
@@ -735,7 +740,7 @@ func (cs *ChainStore) GetRandomness(ctx context.Context, pts *types.TipSet, tick
 			t := mtb.Tickets[0]
 
 			rval := t.VDFResult
-			for i := 0; i < nv; i++ {
+			for i := int64(0); i < nv; i++ {
 				h := sha256.Sum256(rval)
 				rval = h[:]
 			}
diff --git a/chain/sync.go b/chain/sync.go
index 3af36f57f..b9de70f4e 100644
--- a/chain/sync.go
+++ b/chain/sync.go
@@ -313,7 +313,7 @@ func (syncer *Syncer) ValidateTipSet(ctx context.Context, fts *store.FullTipSet)
 
 	for _, b := range fts.Blocks {
 		if err := syncer.ValidateBlock(ctx, b); err != nil {
-			return err
+			return xerrors.Errorf("validating block %s: %w", b.Cid(), err)
 		}
 	}
 	return nil
@@ -440,7 +440,8 @@ func (syncer *Syncer) ValidateBlock(ctx context.Context, b *types.FullBlock) err
 		return xerrors.Errorf("miner created a block but was not a winner")
 	}
 
-	vmi, err := vm.NewVM(stateroot, h.Height, h.Miner, syncer.store)
+	r := vm.NewChainRand(syncer.store, baseTs, h.Tickets)
+	vmi, err := vm.NewVM(stateroot, h.Height, r, h.Miner, syncer.store)
 	if err != nil {
 		return xerrors.Errorf("failed to instantiate VM: %w", err)
 	}
diff --git a/chain/vm/vm.go b/chain/vm/vm.go
index 644203564..f5c8a21b1 100644
--- a/chain/vm/vm.go
+++ b/chain/vm/vm.go
@@ -67,14 +67,8 @@ func (vmc *VMContext) Message() *types.Message {
 }
 
 func (vmc *VMContext) GetRandomness(height uint64) ([]byte, aerrors.ActorError) {
-	hts := vmc.vm.cs.GetHeaviestTipSet()
 
-	relHeight := int(hts.Height() - height)
-	if relHeight < 0 {
-		return nil, aerrors.Newf(1, "negative relHeight in GetRandomness: %d", relHeight)
-	}
-
-	res, err := vmc.vm.cs.GetRandomness(vmc.ctx, hts, nil, relHeight)
+	res, err := vmc.vm.rand.GetRandomness(vmc.ctx, int64(height))
 	if err != nil {
 		return nil, aerrors.Escalate(err, "could not get randomness")
 	}
@@ -294,9 +288,10 @@ type VM struct {
 	blockHeight uint64
 	blockMiner  address.Address
 	inv         *invoker
+	rand        Rand
 }
 
-func NewVM(base cid.Cid, height uint64, maddr address.Address, cs *store.ChainStore) (*VM, error) {
+func NewVM(base cid.Cid, height uint64, r Rand, maddr address.Address, cs *store.ChainStore) (*VM, error) {
 	buf := bufbstore.NewBufferedBstore(cs.Blockstore())
 	cst := hamt.CSTFromBstore(buf)
 	state, err := state.LoadStateTree(cst, base)
@@ -313,9 +308,33 @@ func NewVM(base cid.Cid, height uint64, maddr address.Address, cs *store.ChainSt
 		blockHeight: height,
 		blockMiner:  maddr,
 		inv:         newInvoker(),
+		rand:        r,
 	}, nil
 }
 
+type Rand interface {
+	GetRandomness(ctx context.Context, h int64) ([]byte, error)
+}
+
+type chainRand struct {
+	cs      *store.ChainStore
+	pts     *types.TipSet
+	tickets []*types.Ticket
+}
+
+func NewChainRand(cs *store.ChainStore, pts *types.TipSet, tickets []*types.Ticket) Rand {
+	return &chainRand{
+		cs:      cs,
+		pts:     pts,
+		tickets: tickets,
+	}
+}
+
+func (cr *chainRand) GetRandomness(ctx context.Context, h int64) ([]byte, error) {
+	lb := (int64(cr.pts.Height()) + int64(len(cr.tickets))) - h
+	return cr.cs.GetRandomness(ctx, cr.pts, cr.tickets, lb)
+}
+
 type ApplyRet struct {
 	types.MessageReceipt
 	ActorErr aerrors.ActorError
diff --git a/go.sum b/go.sum
index f2f20f3b9..a4bfa81ef 100644
--- a/go.sum
+++ b/go.sum
@@ -68,7 +68,6 @@ github.com/eapache/go-resiliency v1.1.0/go.mod h1:kFI+JgMyC7bLPUVY133qvEBtVayf5m
 github.com/eapache/go-xerial-snappy v0.0.0-20180814174437-776d5712da21/go.mod h1:+020luEh2TKB4/GOp8oxxtq0Daoen/Cii55CzbTV6DU=
 github.com/eapache/queue v1.1.0/go.mod h1:6eCeP0CKFpHLu8blIFXhExK/dRa7WDZfr6jVFPTqq+I=
 github.com/fatih/color v1.6.0/go.mod h1:Zm6kSWBoL9eyXnKyktHP6abPY2pDugNf5KwzbycvMj4=
-github.com/filecoin-project/go-amt-ipld v0.0.0-20190917221444-2ed85149c65d/go.mod h1:lKjJYPg2kwbav5f78i5YA8kGccnZn18IySbpneXvaQs=
 github.com/filecoin-project/go-amt-ipld v0.0.0-20190919045431-3650716fff16 h1:NzojcJU1VbS6zdLG13JMYis/cQy/MrN3rxmZRq56jKA=
 github.com/filecoin-project/go-amt-ipld v0.0.0-20190919045431-3650716fff16/go.mod h1:lKjJYPg2kwbav5f78i5YA8kGccnZn18IySbpneXvaQs=
 github.com/filecoin-project/go-leb128 v0.0.0-20190212224330-8d79a5489543 h1:aMJGfgqe1QDhAVwxRg5fjCRF533xHidiKsugk7Vvzug=
diff --git a/node/impl/full/chain.go b/node/impl/full/chain.go
index 94fb5717e..a9e0482c5 100644
--- a/node/impl/full/chain.go
+++ b/node/impl/full/chain.go
@@ -45,7 +45,7 @@ func (a *ChainAPI) ChainHead(context.Context) (*types.TipSet, error) {
 }
 
 func (a *ChainAPI) ChainGetRandomness(ctx context.Context, pts *types.TipSet, tickets []*types.Ticket, lb int) ([]byte, error) {
-	return a.Chain.GetRandomness(ctx, pts, tickets, lb)
+	return a.Chain.GetRandomness(ctx, pts, tickets, int64(lb))
 }
 
 func (a *ChainAPI) ChainWaitMsg(ctx context.Context, msg cid.Cid) (*api.MsgWait, error) {