no internal frame cache for token 0

Hi,
I'm testing a connection with an old gateway using Semtech Packet Forwarder 1.0
The uplink messages work fine but I don't receive the downlink messages.
On log of the chirpstack gateway bridge I can read the following message:

error="no internal frame cache for token 0"

According to the spec v1, the PULL_RESP has token=0.
On the code I can see:

if p.ProtocolVersion != ProtocolVersion1 {
		// these two bytes are unused in ProtocolVersion1
		binary.LittleEndian.PutUint16(out[1:3], p.RandomToken)
}

But I don't find the same check when unmarshall the TX_ACK message.

Could it be a bug ?