Namespace prefix g on id is not defined

Hello, I want to make custom xml for google merchant feed. When I paste google tamplate, I got error as in topic. Here’s an example:

<?xml version="1.0"?>
<rss xmlns:g="https://base.google.com/ns/1.0" version="2.0">
	<channel>
		<title>Example - Google Store</title>

	<!-- BEGIN LOOP -->
	<item>
		<g:id>{ID}</g:id>

	</item>
	<!-- END LOOP -->
	</channel>