The CompressionThreshold parameter should be checked on the compressed packet, not on the original packet.

The intent of the parameter is to fix a minimum MAC payload, as some MAC types need a minimum payload size. E.g., ContikiMAC sets this to 16 bytes.
Obviously this limit should be smaller than the minimum IP packet (40 bytes).

The actual code checks the packet size *before* the compression, leading to a possible violation. As an example, the smallest IP packet (header only, 40 bytes) can be compressed to 6 bytes, which is smaller than the limit.

The solution is to check the packet *after* the compression.