diff --git a/runner/payload/simulator/simulatorstats/types.go b/runner/payload/simulator/simulatorstats/types.go
index 0285b908..b65b7198 100644
--- a/runner/payload/simulator/simulatorstats/types.go
+++ b/runner/payload/simulator/simulatorstats/types.go
@@ -27,9 +27,12 @@ func (o OpcodeStats) Round() OpcodeStats {
 }
 
 func (o OpcodeStats) Add(other OpcodeStats) OpcodeStats {
-	result := make(OpcodeStats)
+	result := make(OpcodeStats, len(o)+len(other))
+	for opcode, count := range o {
+		result[opcode] = count
+	}
 	for opcode, count := range other {
-		result[opcode] = o[opcode] + count
+		result[opcode] += count
 	}
 	return result
 }
@@ -43,9 +46,12 @@ func (o OpcodeStats) Pow(n float64) OpcodeStats {
 }
 
 func (o OpcodeStats) Sub(other OpcodeStats) OpcodeStats {
-	result := make(OpcodeStats)
+	result := make(OpcodeStats, len(o)+len(other))
+	for opcode, count := range o {
+		result[opcode] = count
+	}
 	for opcode, count := range other {
-		result[opcode] = o[opcode] - count
+		result[opcode] -= count
 	}
 	return result
 }
diff --git a/runner/payload/simulator/simulatorstats/types_test.go b/runner/payload/simulator/simulatorstats/types_test.go
new file mode 100644
index 00000000..0d2b0ea1
--- /dev/null
+++ b/runner/payload/simulator/simulatorstats/types_test.go
@@ -0,0 +1,76 @@
+package simulatorstats
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestOpcodeStatsAdd_UnionOfKeys(t *testing.T) {
+	a := OpcodeStats{"A": 1, "B": 2}
+	b := OpcodeStats{"B": 10, "C": 100}
+
+	got := a.Add(b)
+
+	require.Equal(t, 1.0, got["A"], "key only in receiver must be preserved")
+	require.Equal(t, 12.0, got["B"], "shared key must sum")
+	require.Equal(t, 100.0, got["C"], "key only in arg must be preserved")
+	require.Len(t, got, 3)
+}
+
+func TestOpcodeStatsAdd_EmptyOther(t *testing.T) {
+	a := OpcodeStats{"A": 1, "B": 2}
+	got := a.Add(OpcodeStats{})
+	require.Equal(t, 1.0, got["A"])
+	require.Equal(t, 2.0, got["B"])
+	require.Len(t, got, 2)
+}
+
+func TestOpcodeStatsAdd_EmptyReceiver(t *testing.T) {
+	got := OpcodeStats{}.Add(OpcodeStats{"A": 1, "B": 2})
+	require.Equal(t, 1.0, got["A"])
+	require.Equal(t, 2.0, got["B"])
+	require.Len(t, got, 2)
+}
+
+func TestOpcodeStatsSub_UnionOfKeys(t *testing.T) {
+	a := OpcodeStats{"A": 10, "B": 20}
+	b := OpcodeStats{"B": 5, "C": 100}
+
+	got := a.Sub(b)
+
+	require.Equal(t, 10.0, got["A"], "key only in receiver must be preserved")
+	require.Equal(t, 15.0, got["B"], "shared key must subtract")
+	require.Equal(t, -100.0, got["C"], "key only in arg must be included (negated)")
+	require.Len(t, got, 3)
+}
+
+func TestOpcodeStatsSub_EmptyOther(t *testing.T) {
+	a := OpcodeStats{"A": 10, "B": 20}
+	got := a.Sub(OpcodeStats{})
+	require.Equal(t, 10.0, got["A"])
+	require.Equal(t, 20.0, got["B"])
+	require.Len(t, got, 2)
+}
+
+func TestStatsSubAdd_FirstTxBlockCountsIncludePrecompiles(t *testing.T) {
+	base := &Stats{
+		Precompiles: OpcodeStats{"ecrecover": 0.5, "bls12381MapG2": 1.0},
+		Opcodes:     OpcodeStats{"KECCAK256": 10.0},
+	}
+
+	expected := base.Mul(1.0)
+	actual := NewStats()
+
+	blockCounts := expected.Sub(actual).Round()
+
+	require.Equal(t, 1.0, blockCounts.Precompiles["ecrecover"],
+		"precompiles missing in blockCounts means worker txs skip precompile execution")
+	require.Equal(t, 1.0, blockCounts.Precompiles["bls12381MapG2"])
+	require.Equal(t, 10.0, blockCounts.Opcodes["KECCAK256"])
+
+	actual = actual.Add(blockCounts)
+	require.Equal(t, 1.0, actual.Precompiles["ecrecover"],
+		"accumulated actual must remember the keys we added")
+	require.Equal(t, 1.0, actual.Precompiles["bls12381MapG2"])
+}