diff --git a/reference/info/functions/getopt.xml b/reference/info/functions/getopt.xml
index 799cb41d27..a21bfa75a4 100644
--- a/reference/info/functions/getopt.xml
+++ b/reference/info/functions/getopt.xml
@@ -34,6 +34,9 @@
        For example, an option string <literal>"x"</literal> recognizes an
        option <literal>-x</literal>.
       </simpara>
+      <simpara>
+       Only a-z, A-Z and 0-9 are allowed.
+      </simpara>
      </listitem>
     </varlistentry>
     <varlistentry>