diff --git a/reference/filesystem/functions/disk-free-space.xml b/reference/filesystem/functions/disk-free-space.xml
index 6f34616941..fef29188dd 100644
--- a/reference/filesystem/functions/disk-free-space.xml
+++ b/reference/filesystem/functions/disk-free-space.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.7 $ -->
+<!-- $Revision: 1.8 $ -->
 <refentry xmlns="http://docbook.org/ns/docbook" xml:id="function.disk-free-space">
  <refnamediv>
   <refname>disk_free_space</refname>
@@ -29,6 +29,13 @@
       <para>
        A directory of the filesystem or disk partition.
       </para>
+      <note>
+       <para>
+        Given a file name instead of a directory, the behaviour of the
+        function is unspecified and may differ between operating systems and
+        PHP versions.
+       </para>
+      </note>
      </listitem>
     </varlistentry>
    </variablelist>