i can't offline filtering from pcap file

[frkn4129]

I want to filter incoming packets and write them to a new file. But I can't filter. SetBPFFilter function returns empty.
what should I do ?

		if handle, err := pcap.**OpenOffline**(/home/x/x/example.pcap); err != nil {
			panic(err)
		} else if err := **handle.SetBPFFilter(" port 80");** err != nil { // Filter 
			panic(err)
		} else {
			packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
			for packet := range packetSource.Packets() {
				fmt.Println(packet) // Do something with a packet here.
				writeFile(packet, fmt.Sprint(t.Id))

			}
		}
	}