diff --git a/reference/exec/functions/shell-exec.xml b/reference/exec/functions/shell-exec.xml
index 5595f01c247e..bbcf23cef21d 100644
--- a/reference/exec/functions/shell-exec.xml
+++ b/reference/exec/functions/shell-exec.xml
@@ -45,8 +45,9 @@
  <refsect1 role="returnvalues">
   &reftitle.returnvalues;
   <para>
-   The output from the executed command or &null; if an error occurred or the
-   command produces no output.
+   The output from the executed command, &false; if the pipe can't be established
+   or &null; if an error occurred or the command produces no output. When &false;
+   is returned, an error of level E_WARNING is also generated.
   </para>
   <note>
    <para>