diff --git a/merkletree_test.go b/merkletree_test.go
index cf19d81..361e092 100644
--- a/merkletree_test.go
+++ b/merkletree_test.go
@@ -301,7 +301,7 @@ var tests = []struct {
 			[]byte("Bar"),
 			[]byte("Foo"),
 		},
-		root:   _byteArray("bd5d1211ddae4a38d2627d8d31922315d929857541ef40cfa5a3f9696e3bdd35"),
+		root:   _byteArray("93478030114bd1f4880ac24dc764f7f60543e455905fdd7a7de5199b205153e5"),
 		dot:    `digraph MerkleTree {rankdir = TB;node [shape=rectangle margin="0.2,0.2"];"Foo" [shape=oval];"Foo"->2;2 [label="195e…278a"];2->1;"Bar" [shape=oval];"Bar"->3;3 [label="65a0…1755"];2->3 [style=invisible arrowhead=none];3->1;{rank=same;2;3};1 [label="bd5d…dd35"];}`,
 		sorted: true,
 	},
@@ -318,7 +318,7 @@ var tests = []struct {
 			[]byte("FooBaz"),
 			[]byte("BarBaz"),
 		},
-		root:   _byteArray("b7c3dd02859b9fb386507ad8de3746b152bc3f9c8aa10fed9999c223f05cca7c"),
+		root:   _byteArray("cd9f207c306b2d0587bfec3ce7aed296f5fcc90807db0a5a8f884a98cecbb996"),
 		sorted: true,
 		dot:    "digraph MerkleTree {rankdir = TB;node [shape=rectangle margin=\"0.2,0.2\"];\"Qux\" [shape=oval];\"Qux\"->16;16 [label=\"003d…71bd\"];16->8;\"Foo\" [shape=oval];\"Foo\"->17;17 [label=\"195e…278a\"];16->17 [style=invisible arrowhead=none];17->8;\"FooBar\" [shape=oval];\"FooBar\"->18;18 [label=\"3007…265f\"];17->18 [style=invisible arrowhead=none];18->9;\"Bar\" [shape=oval];\"Bar\"->19;19 [label=\"65a0…1755\"];18->19 [style=invisible arrowhead=none];19->9;\"Baz\" [shape=oval];\"Baz\"->20;20 [label=\"6a08…3c41\"];19->20 [style=invisible arrowhead=none];20->10;\"Quux\" [shape=oval];\"Quux\"->21;21 [label=\"7781…1042\"];20->21 [style=invisible arrowhead=none];21->10;\"FooBaz\" [shape=oval];\"FooBaz\"->22;22 [label=\"9209…fb63\"];21->22 [style=invisible arrowhead=none];22->11;\"Quuz\" [shape=oval];\"Quuz\"->23;23 [label=\"ce1e…f6ea\"];22->23 [style=invisible arrowhead=none];23->11;\"BarBaz\" [shape=oval];\"BarBaz\"->24;24 [label=\"f86c…483b\"];23->24 [style=invisible arrowhead=none];24->12;25 [label=\"0000…0000\"];24->25 [style=invisible arrowhead=none];25->12;26 [label=\"0000…0000\"];25->26 [style=invisible arrowhead=none];26->13;27 [label=\"0000…0000\"];26->27 [style=invisible arrowhead=none];27->13;28 [label=\"0000…0000\"];27->28 [style=invisible arrowhead=none];28->14;29 [label=\"0000…0000\"];28->29 [style=invisible arrowhead=none];29->14;30 [label=\"0000…0000\"];29->30 [style=invisible arrowhead=none];30->15;31 [label=\"0000…0000\"];30->31 [style=invisible arrowhead=none];31->15;{rank=same;16;17;18;19;20;21;22;23;24;25;26;27;28;29;30;31};15 [label=\"070f…81e0\"];15->7;14 [label=\"070f…81e0\"];14->7;13 [label=\"070f…81e0\"];13->6;12 [label=\"eaa6…7b07\"];12->6;11 [label=\"f4e2…59b9\"];11->5;10 [label=\"1bea…1f21\"];10->5;9 [label=\"ff5a…afc1\"];9->4;8 [label=\"0fb6…8e6c\"];8->4;7 [label=\"53da…4530\"];7->3;6 [label=\"812c…f0f7\"];6->3;5 [label=\"e0c3…a1ad\"];5->2;4 [label=\"1e5e…1997\"];4->2;3 [label=\"4f01…3c5c\"];3->1;2 [label=\"4ab6…2203\"];2->1;1 [label=\"1109…6371\"];}",
 	},
diff --git a/multiproof_test.go b/multiproof_test.go
index 5b32e07..e4c1fc8 100644
--- a/multiproof_test.go
+++ b/multiproof_test.go
@@ -37,14 +37,14 @@ func TestMultiProofWithIndices(t *testing.T) {
 
 			// Test proof for all combinations of data.
 			var proof *MultiProof
-			combinations := 1<<len(test.data) - 1
+			combinations := 1<<len(tree.Data) - 1
 			for j := 1; j <= combinations; j++ {
 				indices := make([]uint64, 0)
 				items := make([][]byte, 0)
-				for k := 0; k < len(test.data); k++ {
+				for k := 0; k < len(tree.Data); k++ {
 					if (j>>k)&1 == 1 {
 						indices = append(indices, uint64(k))
-						items = append(items, test.data[k])
+						items = append(items, tree.Data[k])
 					}
 				}
 				proof, err = tree.GenerateMultiProofWithIndices(indices)
diff --git a/parameters.go b/parameters.go
index e0b37af..ceb18c9 100644
--- a/parameters.go
+++ b/parameters.go
@@ -43,7 +43,12 @@ func (f parameterFunc) apply(p *parameters) {
 // WithData sets the data for the merkle tree.
 func WithData(data [][]byte) Parameter {
 	return parameterFunc(func(p *parameters) {
-		p.data = data
+		dup := make([][]byte, len(data))
+		for i := range data {
+			dup[i] = make([]byte, len(data[i]))
+			copy(dup[i], data[i])
+		}
+		p.data = dup
 	})
 }