Index: class.c
==================================================================
--- class.c
+++ class.c
@@ -2197,10 +2197,11 @@
     while(i<size && data[i++]);
     if(i==size && data[i-1]) fatal("Malformed CLASS.DEF lump\n");
     if(messages[n]) fatal("Duplicate message number %d\n",n+256);
     messages[n]=strdup(p);
     if(!messages[n]) fatal("Allocation failed\n");
+    if(n>=undef_message) undef_message=n+1;
   }
   free(data);
 }
 
 static void parse_order_block(void) {