diff --git a/functions/math.xml b/functions/math.xml
index 0da97c37c9..390ae1c80a 100644
--- a/functions/math.xml
+++ b/functions/math.xml
@@ -923,7 +923,9 @@ $randval = mt_rand();
      </funcprototype>
     </funcsynopsis>
     <para>
-     Returns base raised to the power of exp.
+     Returns <parameter>base</parameter> raised to the power of 
+     <parameter>exp</parameter>. If this can't be computed, 
+     <function>exp</function> will return <literal>NAN</literal>.
     </para>
     <para> 
      See also <function>exp</function>.