diff --git a/reference/image/functions/image-type-to-extension.xml b/reference/image/functions/image-type-to-extension.xml
index 133fc00b5f..f7f8243037 100644
--- a/reference/image/functions/image-type-to-extension.xml
+++ b/reference/image/functions/image-type-to-extension.xml
@@ -10,7 +10,7 @@
   <methodsynopsis>
    <type>string</type><methodname>image_type_to_extension</methodname>
    <methodparam><type>int</type><parameter>imagetype</parameter></methodparam>
-   <methodparam choice="opt"><type>bool</type><parameter>include_dot</parameter></methodparam>
+   <methodparam choice="opt"><type>bool</type><parameter>include_dot</parameter><initializer>&true;</initializer></methodparam>
   </methodsynopsis>
   <para>
    Returns the extension for the given <literal>IMAGETYPE_XXX</literal>