Unable to decode LinkType 20

i use tcpdump to get a tcp package,and then save a cap file,
but i use gopacket parsing this cap file,following error occurs:
"Packet decoding error: Unable to decode LinkType 20"

my code:
handle, err = pcap.OpenOffline(capFile)

packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
//go print_counter()
for packet := range packetSource.Packets() {
fmt.Println(packet) //**Packet decoding error: Unable to decode LinkType 20**
printPacketInfo(packet)
}

... ...

tcpLayer := packet.Layer(layers.LayerTypeTCP) //**Causes an error here ,is null**