diff --git a/gdpr/protect_device_id.go b/gdpr/protect_device_id.go
index bf8ea15..949ba53 100644
--- a/gdpr/protect_device_id.go
+++ b/gdpr/protect_device_id.go
@@ -10,9 +10,12 @@ const (
 
 // ProtectDeviceID hides last two character from passed device id and returns string with protected value
 func ProtectDeviceID(val string) string {
-	lowered := strings.ToLower(val)
+	if val == "" {
+		return val
+	}
 
-	if val == "" || lowered == unknownValue || lowered == nonAvailableValue {
+	lowered := strings.ToLower(val)
+	if lowered == unknownValue || lowered == nonAvailableValue {
 		return val
 	}
 
diff --git a/gdpr/protect_device_id_test.go b/gdpr/protect_device_id_test.go
index 53f71c1..fb94f80 100644
--- a/gdpr/protect_device_id_test.go
+++ b/gdpr/protect_device_id_test.go
@@ -86,3 +86,48 @@ func BenchmarkProtectDeviceID(b *testing.B) {
 		ProtectDeviceID(strconv.Itoa(i))
 	}
 }
+
+func BenchmarkProtectDeviceID_Empty(b *testing.B) {
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		ProtectDeviceID("")
+	}
+}
+
+func BenchmarkProtectDeviceID_NA_Mixed(b *testing.B) {
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		ProtectDeviceID("N/a")
+	}
+}
+
+func BenchmarkProtectDeviceID_NA_Lower(b *testing.B) {
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		ProtectDeviceID("n/a")
+	}
+}
+
+func BenchmarkProtectDeviceID_Unknown_Mixed(b *testing.B) {
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		ProtectDeviceID("Unknown")
+	}
+}
+
+func BenchmarkProtectDeviceID_Unknown_Lower(b *testing.B) {
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		ProtectDeviceID("unknown")
+	}
+}