The attached program has two header field types ip_hdr_prefix and udp_hdr, both has a field with name len. When accessing ip#len, the compiler errored:

error: bugreport_fieldname.dpt: Cannot unify
 {int<<16>> sport; int<<16>> dport; int<<16>> len; int<<16>> csum}
 with
 {int<<8>> v_ihl; int<<8>> tos; int<<16>> len; int<<16>> id; int<<16>> flags_frag; int<<8>> ttl; int<<8>> proto; int<<16>> csum}

Looks like the second "len" in udp_hdr caused the problem; changing it to some other field name will fix the issue.

bugreport_fieldname.dpt