diff --git a/ntp.go b/ntp.go
index 53be5a4..ba47e43 100644
--- a/ntp.go
+++ b/ntp.go
@@ -548,11 +548,18 @@ func toInterval(t int8) time.Duration {
 }
 
 func kissCode(id uint32) string {
+	isPrintable := func(ch byte) bool { return ch >= 32 && ch <= 126 }
+
 	b := []byte{
 		byte(id >> 24),
 		byte(id >> 16),
 		byte(id >> 8),
 		byte(id),
 	}
+	for _, ch := range b {
+		if !isPrintable(ch) {
+			return ""
+		}
+	}
 	return string(b)
 }
diff --git a/ntp_test.go b/ntp_test.go
index 86f4b55..a7ea62a 100644
--- a/ntp_test.go
+++ b/ntp_test.go
@@ -326,6 +326,10 @@ func TestKissCode(t *testing.T) {
 		{0x52415445, "RATE"},
 		{0x524d4f54, "RMOT"},
 		{0x53544550, "STEP"},
+		{0x01010101, ""},
+		{0xfefefefe, ""},
+		{0x01544450, ""},
+		{0x41544401, ""},
 	}
 	for _, c := range codes {
 		assert.Equal(t, kissCode(c.id), c.str)