diff --git a/merkle.go b/merkle.go
index 493b0d1..4fa73cb 100644
--- a/merkle.go
+++ b/merkle.go
@@ -214,8 +214,9 @@ func (t *Tree) RootAndProof() ([]byte, [][]byte) {
 	return ephemeralNode.value, ephemeralProof
 }
 
-func (t *Tree) GetParkedNodes() [][]byte {
-	var ret [][]byte
+// GetParkedNodes appends parked nodes from all layers
+// starting with the base layer to the `ret`.
+func (t *Tree) GetParkedNodes(ret [][]byte) [][]byte {
 	layer := t.baseLayer
 	for {
 		ret = append(ret, layer.parking.value)
diff --git a/merkle_test.go b/merkle_test.go
index 0288f3a..4ed1e81 100644
--- a/merkle_test.go
+++ b/merkle_test.go
@@ -438,22 +438,22 @@ func TestTree_GetParkedNodes(t *testing.T) {
 	r.NoError(tree.AddLeaf([]byte{0}))
 	r.EqualValues(
 		[][]byte{{0}},
-		tree.GetParkedNodes())
+		tree.GetParkedNodes(nil))
 
 	r.NoError(tree.AddLeaf([]byte{1}))
 	r.EqualValues(
 		[][]byte{nil, decode(r, "b413f47d13ee2fe6c845b2ee141af81de858df4ec549a58b7970bb96645bc8d2")},
-		tree.GetParkedNodes())
+		tree.GetParkedNodes(nil))
 
 	r.NoError(tree.AddLeaf([]byte{2}))
 	r.EqualValues(
 		[][]byte{{2}, decode(r, "b413f47d13ee2fe6c845b2ee141af81de858df4ec549a58b7970bb96645bc8d2")},
-		tree.GetParkedNodes())
+		tree.GetParkedNodes(nil))
 
 	r.NoError(tree.AddLeaf([]byte{3}))
 	r.EqualValues(
 		[][]byte{nil, nil, decode(r, "7699a4fdd6b8b6908a344f73b8f05c8e1400f7253f544602c442ff5c65504b24")},
-		tree.GetParkedNodes())
+		tree.GetParkedNodes(nil))
 }
 
 func TestTree_SetParkedNodes(t *testing.T) {
@@ -464,21 +464,21 @@ func TestTree_SetParkedNodes(t *testing.T) {
 	r.NoError(tree.SetParkedNodes([][]byte{{0}}))
 	r.NoError(tree.AddLeaf([]byte{1}))
 	parkedNodes := [][]byte{nil, decode(r, "b413f47d13ee2fe6c845b2ee141af81de858df4ec549a58b7970bb96645bc8d2")}
-	r.EqualValues(parkedNodes, tree.GetParkedNodes())
+	r.EqualValues(parkedNodes, tree.GetParkedNodes(nil))
 
 	tree, err = NewTreeBuilder().Build()
 	r.NoError(err)
 	r.NoError(tree.SetParkedNodes(parkedNodes))
 	r.NoError(tree.AddLeaf([]byte{2}))
 	parkedNodes = [][]byte{{2}, decode(r, "b413f47d13ee2fe6c845b2ee141af81de858df4ec549a58b7970bb96645bc8d2")}
-	r.EqualValues(parkedNodes, tree.GetParkedNodes())
+	r.EqualValues(parkedNodes, tree.GetParkedNodes(nil))
 
 	tree, err = NewTreeBuilder().Build()
 	r.NoError(err)
 	r.NoError(tree.SetParkedNodes(parkedNodes))
 	r.NoError(tree.AddLeaf([]byte{3}))
 	parkedNodes = [][]byte{nil, nil, decode(r, "7699a4fdd6b8b6908a344f73b8f05c8e1400f7253f544602c442ff5c65504b24")}
-	r.EqualValues(parkedNodes, tree.GetParkedNodes())
+	r.EqualValues(parkedNodes, tree.GetParkedNodes(nil))
 }
 
 func decode(r *require.Assertions, hexString string) []byte {