diff --git a/peers/sticktable/sticktables_test.go b/peers/sticktable/sticktables_test.go
index 52a0aac..dc086c0 100644
--- a/peers/sticktable/sticktables_test.go
+++ b/peers/sticktable/sticktables_test.go
@@ -1,6 +1,7 @@
 package sticktable
 
 import (
+	"net/netip"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -78,3 +79,49 @@ func TestMarshalUnmarshalMessages(t *testing.T) {
 		}
 	})
 }
+
+// TestEntryUpdateMarshalEncodesData guards against a regression where
+// EntryUpdate.Marshal encoded the data section by calling MapData.Unmarshal
+// instead of Marshal. That produced a zeroed data section on the wire and, as a
+// side effect, mutated the source EntryUpdate's data to match — which hid the
+// bug from a round-trip test that compares against the same (now-mutated) value.
+//
+// This test uses an independent expected value and also asserts the source is
+// left unchanged, so it fails if the data is not actually encoded.
+func TestEntryUpdateMarshalEncodesData(t *testing.T) {
+	def := &Definition{
+		Name:      "reputation",
+		KeyType:   KeyTypeIPv4Address,
+		KeyLength: 4,
+		DataTypes: []DataTypeDefinition{{DataType: DataTypeGPT0}},
+	}
+	key := IPv4AddressKey(netip.MustParseAddr("1.2.3.4"))
+	data := UnsignedIntegerData(42)
+	e := &EntryUpdate{
+		StickTable:        def,
+		WithLocalUpdateID: true,
+		LocalUpdateID:     7,
+		Key:               &key,
+		Data:              []MapData{&data},
+	}
+
+	b := make([]byte, 64)
+	n, err := e.Marshal(b)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Marshalling must not mutate the source data.
+	if data != 42 {
+		t.Fatalf("Marshal mutated source data to %d, want 42", data)
+	}
+
+	// The encoded bytes must decode back to the original value.
+	got := EntryUpdate{StickTable: def, WithLocalUpdateID: true}
+	if _, err := got.Unmarshal(b[:n]); err != nil {
+		t.Fatal(err)
+	}
+	if len(got.Data) != 1 || got.Data[0].String() != "42" {
+		t.Errorf("round-trip data = %v, want [42]", got.Data)
+	}
+}