diff --git a/reference/readline/functions/readline.xml b/reference/readline/functions/readline.xml
index be37b0569c..76c3000d85 100644
--- a/reference/readline/functions/readline.xml
+++ b/reference/readline/functions/readline.xml
@@ -10,7 +10,7 @@
   &reftitle.description;
   <methodsynopsis>
    <type>string</type><methodname>readline</methodname>
-   <methodparam choice="opt"><type>string</type><parameter>prompt</parameter></methodparam>
+   <methodparam choice="opt"><type>string</type><parameter>prompt</parameter><initializer>&null;</initializer></methodparam>
   </methodsynopsis>
   <para>
    Reads a single line from the user. You must add this line to the history