diff --git a/docs/TableGenFundamentals.html b/docs/TableGenFundamentals.html
index 05b2b23..ec87f29 100644
--- a/docs/TableGenFundamentals.html
+++ b/docs/TableGenFundamentals.html
@@ -411,7 +411,8 @@
 <dt><tt>!cast<type>(a)</tt></dt>
   <dd>A symbol of type <em>type</em> obtained by looking up the string 'a' in
 the symbol table.  If the type of 'a' does not match <em>type</em>, TableGen
-aborts with an error. </dd>
+aborts with an error. !cast<string> is a special case in that the argument must
+be an object defined by a 'def' construct.</dd>
 <dt><tt>!nameconcat&lt;type&gt;(a, b)</tt></dt>
   <dd>Shorthand for !cast<type>(!strconcat(a, b))</dd>
 <dt><tt>!subst(a, b, c)</tt></dt>