ModbusTCP - Question

I am trying to decode ModbusTCP packets from pcap file, but i cant

```
if handle, err := pcap.OpenOffline("test.pcap"); err != nil {
panic(err)
} else {
packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
for packet := range packetSource.Packets() {
pkt := handler(packet)
}
}
```

here is the handler()
```
modbusLayer := packet.Layer(layers.LayerTypeModbusTCP)
if modbusLayer != nil {
mod, _ := modbusLayer.(*layers.ModbusTCP)
fmt.Print("MODBUS PACKET")
```

@vgraveto