diff --git a/gdpr/protect_device_id.go b/gdpr/protect_device_id.go
index c90112e..949ba53 100644
--- a/gdpr/protect_device_id.go
+++ b/gdpr/protect_device_id.go
@@ -1,7 +1,11 @@
 package gdpr
 
+import "strings"
+
 const (
-	hiddenRune = '*'
+	hiddenRune        = '*'
+	unknownValue      = "unknown"
+	nonAvailableValue = "n/a"
 )
 
 // ProtectDeviceID hides last two character from passed device id and returns string with protected value
@@ -10,6 +14,11 @@ func ProtectDeviceID(val string) string {
 		return val
 	}
 
+	lowered := strings.ToLower(val)
+	if lowered == unknownValue || lowered == nonAvailableValue {
+		return val
+	}
+
 	r := []rune(val)
 	l := len(r)
 
diff --git a/gdpr/protect_device_id_test.go b/gdpr/protect_device_id_test.go
index 9cdd40f..fb94f80 100644
--- a/gdpr/protect_device_id_test.go
+++ b/gdpr/protect_device_id_test.go
@@ -18,6 +18,36 @@ func Test_ProtectDeviceID(t *testing.T) {
 			value: "",
 			want:  "",
 		},
+		{
+			name:  "unknown value - lower case",
+			value: "unknown",
+			want:  "unknown",
+		},
+		{
+			name:  "unknown value - upper case",
+			value: "UNKNOWN",
+			want:  "UNKNOWN",
+		},
+		{
+			name:  "unknown value - mixed case",
+			value: "unKNowN",
+			want:  "unKNowN",
+		},
+		{
+			name:  "non available value - upper case",
+			value: "N/A",
+			want:  "N/A",
+		},
+		{
+			name:  "non available value - lower case",
+			value: "n/a",
+			want:  "n/a",
+		},
+		{
+			name:  "non available value - mixed case",
+			value: "n/A",
+			want:  "n/A",
+		},
 		{
 			name:  "correct value to protect",
 			value: "some_value",
@@ -56,3 +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")
+	}
+}