/sigh

You should endeavor to follow Postel's law.  In this case, that means (using RFC2119 notation):

If you are creating a "headers" packet, you must include a properly coded varint representing the number of transactions that are included in the packet.  The number of transactions included must be zero.  (The only valid encoding for zero is a single octet: 0x00.  0x05 is right out.)

If you are reading a "headers" packet, you must parse that field as a varint.  You should reject any packets that include a non-zero number of transactions.  You may disconnect the peer that sent it, and attempt to publicly shame the authors of that implementation.