Protocol scan and Host Unreachable

Why is "host unreachable" interpreted the same as "protocol
unreachable"? Is it a bug or do some hosts give that answer for unused
protocols?

nmap-3.10ALPHA7/scan_engine.cc:
   ...
   switch(icmp->icmp_code) {

   case 1: /* Host unreachable */
   case 2: /* pr0t0c0l unreachable */
     if (scantype == IPPROT_SCAN) {
       newstate = PORT_CLOSED;
       ...

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Received on Dec 19 2002