diff --git a/db_test.go b/db_test.go
index a6dbc8e..88b06ef 100644
--- a/db_test.go
+++ b/db_test.go
@@ -755,6 +755,37 @@ func TestDB_Modify(t *testing.T) {
 	require.EqualValues(t, 1, objs[0].ID)
 }
 
+func TestDB_ModifyUpdatesSecondaryIndex(t *testing.T) {
+	t.Parallel()
+
+	db, table, _ := newTestDB(t, tagsIndex)
+
+	txn := db.WriteTxn(table)
+	_, _, err := table.Insert(txn, &testObject{ID: uint64(1), Tags: part.NewSet("foo")})
+	require.NoError(t, err, "Insert failed")
+
+	_, hadOld, err := table.Modify(txn, &testObject{ID: uint64(1)}, func(old, new *testObject) *testObject {
+		return &testObject{
+			ID:   1,
+			Tags: old.Tags.Set("bar"),
+		}
+	})
+	require.NoError(t, err, "Modify failed")
+	require.True(t, hadOld, "expected hadOld to be true")
+
+	rtxn := txn.Commit()
+
+	obj, _, found := table.Get(rtxn, tagsIndex.Query("bar"))
+	require.True(t, found, "expected tags index to include merged tag")
+	require.EqualValues(t, 1, obj.ID)
+	require.True(t, obj.Tags.Has("foo"))
+	require.True(t, obj.Tags.Has("bar"))
+
+	obj, _, found = table.Get(rtxn, tagsIndex.Query("foo"))
+	require.True(t, found, "expected tags index to retain existing tag")
+	require.EqualValues(t, 1, obj.ID)
+}
+
 func TestDB_Revision(t *testing.T) {
 	t.Parallel()
 
diff --git a/lpm_index.go b/lpm_index.go
index 36e3921..2b28a4e 100644
--- a/lpm_index.go
+++ b/lpm_index.go
@@ -312,7 +312,7 @@ func (l *lpmIndexTxn) insert(key index.Key, obj object) (old object, hadOld bool
 }
 
 // modify implements tableIndexTxn.
-func (l *lpmIndexTxn) modify(key index.Key, obj object, mod func(old, new object) object) (old object, hadOld bool, watch <-chan struct{}) {
+func (l *lpmIndexTxn) modify(key index.Key, obj object, mod func(old, new object) object) (old object, newObj object, hadOld bool, watch <-chan struct{}) {
 	panic("LPM index cannot be the primary index")
 }
 
diff --git a/part/part_test.go b/part/part_test.go
index 06f1c32..cd569ae 100644
--- a/part/part_test.go
+++ b/part/part_test.go
@@ -672,7 +672,7 @@ func Test_modify(t *testing.T) {
 
 	txn := tree.Txn()
 	for i := range 1000 {
-		old, hadOld := txn.Modify(key, 123, func(x, _ int) int { return x + 1 })
+		old, _, hadOld := txn.Modify(key, 123, func(x, _ int) int { return x + 1 })
 		require.True(t, hadOld)
 		require.Equal(t, i+1, old)
 	}
diff --git a/part/tree.go b/part/tree.go
index 24ea5e6..3b18897 100644
--- a/part/tree.go
+++ b/part/tree.go
@@ -127,7 +127,7 @@ func (t *Tree[T]) Insert(key []byte, value T) (old T, hadOld bool, tree Tree[T])
 // Returns the old value if it exists.
 func (t *Tree[T]) Modify(key []byte, value T, mod func(T, T) T) (old T, hadOld bool, tree Tree[T]) {
 	txn := t.Txn()
-	old, hadOld = txn.Modify(key, value, mod)
+	old, _, hadOld = txn.Modify(key, value, mod)
 	tree = txn.CommitAndNotify()
 	return
 }
diff --git a/part/txn.go b/part/txn.go
index 4d4936d..d7fa3ed 100644
--- a/part/txn.go
+++ b/part/txn.go
@@ -80,7 +80,7 @@ func (txn *Txn[T]) Insert(key []byte, value T) (old T, hadOld bool) {
 // Returns the old value if it exists and a watch channel that closes when the
 // key changes again.
 func (txn *Txn[T]) InsertWatch(key []byte, value T) (old T, hadOld bool, watch <-chan struct{}) {
-	old, hadOld, watch, txn.root = txn.insert(txn.root, key, value)
+	old, _, hadOld, watch, txn.root = txn.insert(txn.root, key, value)
 	validateTree(txn.root, nil, txn.watches, txn.txnID)
 	if !hadOld {
 		txn.size++
@@ -93,19 +93,19 @@ func (txn *Txn[T]) InsertWatch(key []byte, value T) (old T, hadOld bool, watch <
 
 // Modify a value in the tree. It is up to the
 // caller to not mutate the value in-place and to return a clone.
-// Returns the old value if it exists.
-func (txn *Txn[T]) Modify(key []byte, value T, mod func(T, T) T) (old T, hadOld bool) {
-	old, hadOld, _ = txn.ModifyWatch(key, value, mod)
+// Returns the old value (if it exists) and the new possibly merged value.
+func (txn *Txn[T]) Modify(key []byte, value T, mod func(T, T) T) (old T, newValue T, hadOld bool) {
+	old, newValue, hadOld, _ = txn.ModifyWatch(key, value, mod)
 	return
 }
 
 // Modify a value in the tree. If the key does not exist the modify
 // function is called with the zero value for T. It is up to the
 // caller to not mutate the value in-place and to return a clone.
-// Returns the old value if it exists and a watch channel that closes
-// when the key changes again.
-func (txn *Txn[T]) ModifyWatch(key []byte, value T, mod func(T, T) T) (old T, hadOld bool, watch <-chan struct{}) {
-	old, hadOld, watch, txn.root = txn.modify(txn.root, key, value, mod)
+// Returns the old value (if it exists) and the new possibly merged value,
+// and a watch channel that closes when the key changes again.
+func (txn *Txn[T]) ModifyWatch(key []byte, value T, mod func(T, T) T) (old T, newValue T, hadOld bool, watch <-chan struct{}) {
+	old, newValue, hadOld, watch, txn.root = txn.modify(txn.root, key, value, mod)
 	validateTree(txn.root, nil, txn.watches, txn.txnID)
 	if !hadOld {
 		txn.size++
@@ -243,17 +243,18 @@ func (txn *Txn[T]) cloneNode(n *header[T]) *header[T] {
 	return n
 }
 
-func (txn *Txn[T]) insert(root *header[T], key []byte, value T) (oldValue T, hadOld bool, watch <-chan struct{}, newRoot *header[T]) {
+func (txn *Txn[T]) insert(root *header[T], key []byte, value T) (oldValue T, newValue T, hadOld bool, watch <-chan struct{}, newRoot *header[T]) {
 	return txn.modify(root, key, value, nil)
 }
 
-func (txn *Txn[T]) modify(root *header[T], key []byte, newValue T, mod func(T, T) T) (oldValue T, hadOld bool, watch <-chan struct{}, newRoot *header[T]) {
+func (txn *Txn[T]) modify(root *header[T], key []byte, newValue T, mod func(T, T) T) (oldValue T, newValueOut T, hadOld bool, watch <-chan struct{}, newRoot *header[T]) {
 	txn.dirty = true
 	fullKey := key
+	newValueOut = newValue
 
 	if root == nil {
 		leaf := newLeaf(txn.opts, key, fullKey, newValue)
-		return oldValue, false, leaf.watch, leaf.self()
+		return oldValue, newValueOut, false, leaf.watch, leaf.self()
 	}
 
 	// Start recursing from the root to find the insertion point.
@@ -323,7 +324,8 @@ func (txn *Txn[T]) modify(root *header[T], key []byte, newValue T, mod func(T, T
 			}
 			watch = leaf.watch
 			if mod != nil {
-				leaf.value = mod(oldValue, newValue)
+				newValueOut = mod(oldValue, newValue)
+				leaf.value = newValueOut
 			} else {
 				leaf.value = newValue
 			}
diff --git a/part_index.go b/part_index.go
index 7bde0da..c227a98 100644
--- a/part_index.go
+++ b/part_index.go
@@ -321,7 +321,7 @@ func (r *partIndexTxn) len() int {
 }
 
 // modify implements tableIndexTxn.
-func (r *partIndexTxn) modify(key index.Key, obj object, mod func(old, new object) object) (old object, hadOld bool, watch <-chan struct{}) {
+func (r *partIndexTxn) modify(key index.Key, obj object, mod func(old, new object) object) (old object, newObj object, hadOld bool, watch <-chan struct{}) {
 	return r.tx.ModifyWatch(key, obj, mod)
 }
 
diff --git a/quick_test.go b/quick_test.go
index 79d58c2..0b26c85 100644
--- a/quick_test.go
+++ b/quick_test.go
@@ -120,7 +120,7 @@ func TestDB_Quick(t *testing.T) {
 
 	numInserted, numRemoved := 0, 0
 
-	check := func(a, b string, remove bool) bool {
+	check := func(a, b string, remove bool, useModify bool) bool {
 		txn := db.WriteTxn(table)
 		if remove {
 			key := pickRandom()
@@ -152,13 +152,31 @@ func TestDB_Quick(t *testing.T) {
 
 		getObj, _, getWatch, getFound := table.GetWatch(txn, aIndex.Query(a))
 
-		old, hadOld, err := table.Insert(txn, quickObj{a, b})
-		require.NoError(t, err, "Insert")
+		expected, found := values[a]
+		expectedB := b
+
+		var (
+			old    quickObj
+			hadOld bool
+			err    error
+		)
+		if useModify {
+			old, hadOld, err = table.Modify(txn, quickObj{a, b}, func(old, new quickObj) quickObj {
+				new.B = old.B + new.B
+				return new
+			})
+			require.NoError(t, err, "Modify")
+			if found {
+				expectedB = expected + b
+			}
+		} else {
+			old, hadOld, err = table.Insert(txn, quickObj{a, b})
+			require.NoError(t, err, "Insert")
+		}
 		numInserted++
 
 		require.Equal(t, getFound, hadOld)
 
-		expected, found := values[a]
 		if found {
 			if !hadOld {
 				t.Logf("object was updated but old value not returned")
@@ -174,7 +192,7 @@ func TestDB_Quick(t *testing.T) {
 				return false
 			}
 		}
-		values[a] = b
+		values[a] = expectedB
 
 		if len(values) != table.NumObjects(txn) {
 			t.Logf("wrong object count")
@@ -270,6 +288,7 @@ func TestDB_Quick(t *testing.T) {
 		//
 		// Check against the secondary (non-unique index)
 		//
+		queryB := expectedB
 
 		// Non-unique indexes return the same number of objects as we've inserted.
 		if len(values) != seqLen(table.Prefix(rtxn, bIndex.Query(""))) {
@@ -284,15 +303,15 @@ func TestDB_Quick(t *testing.T) {
 
 		// Get returns the first match, but since the index is non-unique, this might
 		// not be the one that we just inserted.
-		obj, _, found = table.Get(rtxn, bIndex.Query(b))
-		if !found || obj.B != b {
-			t.Logf("Get(%q) via bIndex not found (%v) or wrong B (%q vs %q)", b, found, obj.B, b)
+		obj, _, found = table.Get(rtxn, bIndex.Query(queryB))
+		if !found || obj.B != queryB {
+			t.Logf("Get(%q) via bIndex not found (%v) or wrong B (%q vs %q)", queryB, found, obj.B, queryB)
 			return false
 		}
 
 		found = false
-		for obj := range table.List(rtxn, bIndex.Query(b)) {
-			if obj.B != b {
+		for obj := range table.List(rtxn, bIndex.Query(queryB)) {
+			if obj.B != queryB {
 				t.Logf("List() via bIndex wrong B")
 				return false
 			}
@@ -306,8 +325,8 @@ func TestDB_Quick(t *testing.T) {
 		}
 
 		visited := map[string]struct{}{}
-		for obj := range table.Prefix(rtxn, bIndex.Query(b)) {
-			if !strings.HasPrefix(obj.B, b) {
+		for obj := range table.Prefix(rtxn, bIndex.Query(queryB)) {
+			if !strings.HasPrefix(obj.B, queryB) {
 				t.Logf("Prefix() via bIndex has wrong prefix")
 				return false
 			}
@@ -318,20 +337,20 @@ func TestDB_Quick(t *testing.T) {
 			visited[obj.A] = struct{}{}
 		}
 
-		anyObjs, err = anyTable.Prefix(rtxn, "b", b)
+		anyObjs, err = anyTable.Prefix(rtxn, "b", queryB)
 		require.NoError(t, err, "AnyTable.Prefix")
 		for anyObj := range anyObjs {
 			obj := anyObj.(quickObj)
-			if !strings.HasPrefix(obj.B, b) {
-				t.Logf("AnyTable.Prefix() via bIndex has wrong prefix: %q vs %q", obj.B, b)
+			if !strings.HasPrefix(obj.B, queryB) {
+				t.Logf("AnyTable.Prefix() via bIndex has wrong prefix: %q vs %q", obj.B, queryB)
 				return false
 			}
 		}
 
 		visited = map[string]struct{}{}
-		for obj := range table.LowerBound(rtxn, bIndex.Query(b)) {
-			if cmp.Compare(obj.B, b) < 0 {
-				t.Logf("LowerBound() via bIndex has wrong objects, expected %v >= %v", []byte(obj.B), []byte(b))
+		for obj := range table.LowerBound(rtxn, bIndex.Query(queryB)) {
+			if cmp.Compare(obj.B, queryB) < 0 {
+				t.Logf("LowerBound() via bIndex has wrong objects, expected %v >= %v", []byte(obj.B), []byte(queryB))
 				return false
 			}
 			if _, found := visited[obj.A]; found {
@@ -341,12 +360,12 @@ func TestDB_Quick(t *testing.T) {
 			visited[obj.A] = struct{}{}
 		}
 
-		anyObjs, err = anyTable.LowerBound(rtxn, "b", b)
+		anyObjs, err = anyTable.LowerBound(rtxn, "b", queryB)
 		require.NoError(t, err, "AnyTable.LowerBound")
 		for anyObj := range anyObjs {
 			obj := anyObj.(quickObj)
-			if cmp.Compare(obj.B, b) < 0 {
-				t.Logf("AnyTable.LowerBound() via bIndex has wrong objects, expected %v >= %v", []byte(obj.B), []byte(b))
+			if cmp.Compare(obj.B, queryB) < 0 {
+				t.Logf("AnyTable.LowerBound() via bIndex has wrong objects, expected %v >= %v", []byte(obj.B), []byte(queryB))
 				return false
 			}
 		}
@@ -371,7 +390,7 @@ func TestDB_Quick(t *testing.T) {
 		// than the default quick value generation to hit the more interesting cases
 		// often.
 		Values: func(args []reflect.Value, rand *rand.Rand) {
-			if len(args) != 3 {
+			if len(args) != 4 {
 				panic("unexpected args count")
 			}
 			for i := range args[:2] {
@@ -382,6 +401,8 @@ func TestDB_Quick(t *testing.T) {
 			}
 			// Remove 33% of the time
 			args[2] = reflect.ValueOf(rand.Intn(3) == 1)
+			// Use Modify 50% of the time
+			args[3] = reflect.ValueOf(rand.Intn(2) == 0)
 		},
 	}))
 
diff --git a/table.go b/table.go
index 3ce8a5d..c3b6dcd 100644
--- a/table.go
+++ b/table.go
@@ -486,7 +486,7 @@ func (t *genTable[Obj]) InsertWatch(txn WriteTxn, obj Obj) (oldObj Obj, hadOld b
 
 func (t *genTable[Obj]) Modify(txn WriteTxn, obj Obj, merge func(old, new Obj) Obj) (oldObj Obj, hadOld bool, err error) {
 	mergeObjects := func(old object, new object) object {
-		new.data = merge(old.data.(Obj), obj)
+		new.data = merge(old.data.(Obj), new.data.(Obj))
 		return new
 	}
 	var old object
diff --git a/types.go b/types.go
index e9fd365..8237aae 100644
--- a/types.go
+++ b/types.go
@@ -414,7 +414,7 @@ type tableIndexTxn interface {
 	tableIndex
 
 	insert(key index.Key, obj object) (old object, hadOld bool, watch <-chan struct{})
-	modify(key index.Key, obj object, mod func(old, new object) object) (old object, hadOld bool, watch <-chan struct{})
+	modify(key index.Key, obj object, mod func(old, new object) object) (old object, new object, hadOld bool, watch <-chan struct{})
 	delete(key index.Key) (old object, hadOld bool)
 	reindex(primaryKey index.Key, old object, new object)
 }
diff --git a/write_txn.go b/write_txn.go
index 6aae11d..091ac0d 100644
--- a/write_txn.go
+++ b/write_txn.go
@@ -139,7 +139,9 @@ func (txn *writeTxnState) modify(meta TableMeta, guardRevision Revision, newData
 	if merge == nil {
 		oldObj, oldExists, watch = idIndexTxn.insert(idKey, obj)
 	} else {
-		oldObj, oldExists, watch = idIndexTxn.modify(idKey, obj, merge)
+		// Insert the object into the primary index. This returns the merged new
+		// object which we'll then insert into the secondary indexes.
+		oldObj, obj, oldExists, watch = idIndexTxn.modify(idKey, obj, merge)
 	}
 
 	// Sanity check: is the same object being inserted back and thus the