diff --git a/CHANGELOG.md b/CHANGELOG.md
index 192fa556af3..a9582a28257 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -24,6 +24,7 @@
 - fix(eth): handle nil address in trace_filter for failed contract creates ([filecoin-project/lotus#13549](https://github.com/filecoin-project/lotus/pull/13549))
 - fix(gas): stricter bounds for GasEstimateGasPremium lookback ([filecoin-project/lotus#13555](https://github.com/filecoin-project/lotus/pull/13555))
 - fix(api): `StateSearchMsg` should respect `lookbackLimit` [filecoin-project/lotus#13562](https://github.com/filecoin-project/lotus/pull/13562)
+- fix(ecfinality): account for null rounds in EC finality calculator chain walk, aligning with FRC-0089 theoretical model and fixing depth-to-height conversion ([filecoin-project/lotus#13565](https://github.com/filecoin-project/lotus/pull/13565))
 
 # Node and Miner v1.35.0 / 2026-02-19
 
diff --git a/chain/ecfinality/cache.go b/chain/ecfinality/cache.go
index 30a683c9b15..87d8af61d7d 100644
--- a/chain/ecfinality/cache.go
+++ b/chain/ecfinality/cache.go
@@ -116,18 +116,28 @@ func (c *ECFinalityCache) GetFinalizedTipSet(ctx context.Context) (*types.TipSet
 	return s.FinalizedTipSet, nil
 }
 
-// walkChain walks back from head collecting block counts for the calculator.
-// Each LoadTipSet call typically hits the ChainStore's ARC cache.
+// walkChain walks back from head collecting block counts per epoch for the
+// calculator. Null rounds (epochs with no blocks) are included as 0 entries
+// so that the calculator sees the real timeline and the returned array depth
+// corresponds directly to epoch height differences. Each LoadTipSet call
+// typically hits the ChainStore's ARC cache.
 func (c *ECFinalityCache) walkChain(ctx context.Context, head *types.TipSet) ([]int, error) {
 	needed := c.windowSize
 	chain := make([]int, 0, needed)
 	ts := head
-	for len(chain) < needed {
+	for {
 		chain = append(chain, len(ts.Cids()))
+		if len(chain) >= needed {
+			break
+		}
 		parent, err := c.cs.LoadTipSet(ctx, ts.Parents())
 		if err != nil {
 			return nil, err
 		}
+		// Insert 0 entries for null rounds between this tipset and its parent.
+		for nulls := int(ts.Height()-parent.Height()) - 1; nulls > 0 && len(chain) < needed; nulls-- {
+			chain = append(chain, 0)
+		}
 		ts = parent
 	}
 	// Reverse to chronological order (oldest first).
diff --git a/chain/ecfinality/calculator.go b/chain/ecfinality/calculator.go
index 50b07fefcec..cb0e15f0640 100644
--- a/chain/ecfinality/calculator.go
+++ b/chain/ecfinality/calculator.go
@@ -22,9 +22,9 @@ const (
 	// BisectLow and BisectHigh define the search range for the bisect algorithm
 	// that finds the epoch depth at which the finality guarantee is met. A low
 	// bound of 3 avoids evaluating trivially shallow depths; a high bound of
-	// 200 accommodates degraded chains that take longer to finalize.
+	// 450 accommodates degraded chains that take longer to finalize.
 	BisectLow  = 3
-	BisectHigh = 200
+	BisectHigh = 450
 
 	// DefaultBlocksPerEpoch is the Filecoin mainnet expected block production rate.
 	DefaultBlocksPerEpoch = 5.0
diff --git a/chain/ecfinality/calculator_test.go b/chain/ecfinality/calculator_test.go
index 880eda520df..0024c9a30eb 100644
--- a/chain/ecfinality/calculator_test.go
+++ b/chain/ecfinality/calculator_test.go
@@ -147,11 +147,11 @@ func TestFindThresholdDepth_HealthyChain(t *testing.T) {
 func TestFindThresholdDepth_DegradedChain(t *testing.T) {
 	req := require.New(t)
 
-	// All-2s chain is too degraded to achieve 2^-30 within the bisect
-	// search range (BisectHigh=200), so threshold is not found
+	// All-1s chain is too degraded to achieve 2^-30 within the bisect
+	// search range (BisectHigh=450), so threshold is not found
 	chain := make([]int, 905)
 	for i := range chain {
-		chain[i] = 2
+		chain[i] = 1
 	}
 	guarantee := math.Pow(2, -30)
 
diff --git a/cmd/lotus-shed/finality.go b/cmd/lotus-shed/finality.go
index f97504ddb8c..30693993ff8 100644
--- a/cmd/lotus-shed/finality.go
+++ b/cmd/lotus-shed/finality.go
@@ -83,13 +83,20 @@ machine-readable output of all 900 epochs.`,
 			headEpoch = int(head.Height())
 			readLength := int(policy.ChainFinality) + 5
 			chain = append(chain, len(head.Cids()))
-			for range readLength - 1 {
-				head, err = api.ChainGetTipSet(ctx, head.Parents())
+			for len(chain) < readLength {
+				parent, err := api.ChainGetTipSet(ctx, head.Parents())
 				if err != nil {
 					return err
 				}
-				chain = append(chain, len(head.Cids()))
+				// Insert 0 entries for null rounds between this tipset and its parent.
+				for nulls := int(head.Height()-parent.Height()) - 1; nulls > 0 && len(chain) < readLength; nulls-- {
+					chain = append(chain, 0)
+				}
+				chain = append(chain, len(parent.Cids()))
+				head = parent
 			}
+			// Trim to exact length in case null round insertion overshot.
+			chain = chain[:readLength]
 			// API walk produces most-recent-first; reverse to match the
 			// expected ordering (index 0 = earliest epoch).
 			slices.Reverse(chain)