Index: en_US.ISO8859-1/books/porters-handbook/makefiles/chapter.xml
===================================================================
--- en_US.ISO8859-1/books/porters-handbook/makefiles/chapter.xml
+++ en_US.ISO8859-1/books/porters-handbook/makefiles/chapter.xml
@@ -1632,6 +1632,13 @@
 	closest master site and fetching from there; having multiple
 	sites will go a long way towards helping this effort.</para>
 
+      <important>
+	<para><varname>MASTER_SITES</varname> must not be blank.  It
+	  must point to the actual site hosting the distribution
+	  files.  It cannot point to web archives, or the &os;
+	  distribution files cache sites.</para>
+      </important>
+
       <para>If the original tarball is part of one of the popular
 	archives such as SourceForge, GNU, or Perl CPAN, it may be
 	possible refer to those sites in an easy compact form using