diff --git a/reference/ffi/ffi/cdef.xml b/reference/ffi/ffi/cdef.xml
index 61814ca073..d6652ee817 100644
--- a/reference/ffi/ffi/cdef.xml
+++ b/reference/ffi/ffi/cdef.xml
@@ -12,7 +12,7 @@
   <methodsynopsis>
    <modifier>public</modifier> <modifier>static</modifier> <type>FFI</type><methodname>FFI::cdef</methodname>
    <methodparam choice="opt"><type>string</type><parameter>code</parameter><initializer>""</initializer></methodparam>
-   <methodparam choice="opt"><type>string</type><parameter>lib</parameter><initializer>&null;</initializer></methodparam>
+   <methodparam choice="opt"><type>string</type><parameter>lib</parameter></methodparam>
   </methodsynopsis>
   <para>
    Creates a new FFI object.
@@ -45,6 +45,13 @@
       The name of a shared library file, to be loaded and linked with the
       definitions.
      </para>
+     <note>
+      <para>
+       If <parameter>lib</parameter> is omitted, platforms supporting <literal>RTLD_DEFAULT</literal>
+       attempt to lookup symbols declared in <parameter>code</parameter> in the normal global
+       scope. Other systems will fail to resolve these symbols.
+      </para>
+     </note>
     </listitem>
    </varlistentry>
   </variablelist>