diff --git a/language/oop5/constants.xml b/language/oop5/constants.xml
index 65259835a1..951ac17782 100644
--- a/language/oop5/constants.xml
+++ b/language/oop5/constants.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.6 $ -->
+<!-- $Revision: 1.7 $ -->
  <sect1 id="language.oop5.constants">
   <title>Class Constants</title>
   <para>
@@ -10,6 +10,10 @@
    cannot be accessed from an instance of the object (using
    <literal>$object::constant</literal>).
   </para>
+  <para>
+   The value must be a constant expression, not (for example) a variable, a
+   class member, result of a mathematical operation or a function call.
+  </para>
 
   <example>
    <title>Defining and using a constant</title>