diff --git a/pkg/ueps/packet_test.go b/pkg/ueps/packet_test.go
index 317dc1d..0e24446 100644
--- a/pkg/ueps/packet_test.go
+++ b/pkg/ueps/packet_test.go
@@ -206,3 +206,34 @@ func TestPacket_writeTLV_Ugly(t *testing.T) {
 		t.Errorf("expected length 0, got %d", encodedTLV[1])
 	}
 }
+
+// var e sentinelError = "record not found"; e.Error() == "record not found"
+func TestPacket_sentinelError_Good(t *testing.T) {
+	e := sentinelError("record not found")
+
+	if e.Error() != "record not found" {
+		t.Errorf("expected %q, got %q", "record not found", e.Error())
+	}
+}
+
+// var e sentinelError = ""; e.Error() == "" (empty message is valid sentinel)
+func TestPacket_sentinelError_Bad(t *testing.T) {
+	e := sentinelError("")
+
+	if e.Error() != "" {
+		t.Errorf("expected empty string, got %q", e.Error())
+	}
+}
+
+// var a, b sentinelError = "x", "x"; a == b (sentinel identity: same message = same error)
+func TestPacket_sentinelError_Ugly(t *testing.T) {
+	a := sentinelError("integrity violation")
+	b := sentinelError("integrity violation")
+
+	if a != b {
+		t.Errorf("expected sentinel identity: %q == %q", a, b)
+	}
+	if a.Error() != b.Error() {
+		t.Errorf("expected identical Error() output, got %q vs %q", a.Error(), b.Error())
+	}
+}