diff --git a/adv.go b/adv.go
index 787ff33..a853978 100644
--- a/adv.go
+++ b/adv.go
@@ -93,9 +93,10 @@ func (a *Advertisement) unmarshall(b []byte) error {
 			return errors.New("invalid advertise data")
 		}
 		l, t := b[0], b[1]
-		if len(b) < int(1+l) {
+		if int(l) < 1 || len(b) < int(1+l) {
 			return errors.New("invalid advertise data")
 		}
+
 		d := b[2 : 1+l]
 		switch t {
 		case typeFlags: