diff --git a/reference/outcontrol/functions/ob-start.xml b/reference/outcontrol/functions/ob-start.xml
index a0754580e5..3f27d68bcc 100644
--- a/reference/outcontrol/functions/ob-start.xml
+++ b/reference/outcontrol/functions/ob-start.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.14 $ -->
+<!-- $Revision: 1.15 $ -->
 <!-- splitted from ./en/functions/outcontrol.xml, last change in rev 1.1 -->
   <refentry id="function.ob-start">
    <refnamediv>
@@ -63,6 +63,8 @@
      If the optional parameter <parameter>chunk_size</parameter> is passed, the
      callback function is called on every first newline after
      <parameter>chunk_size</parameter> bytes of output.
+     You can bypass the <parameter>output_callback</parameter> parameter by
+     &null; value.
     </para>
     <para>
      If the optional parameter <parameter>erase</parameter> is set to &false;,