Microdata markup with 'mainEntityOfPage' for Google Article Rich Snippet

The Microdata example of Google’s Article Rich Snippet contains this meta element with Schema.org’s mainEntityOfPage property:

<meta itemscope itemprop="mainEntityOfPage"  itemType="https://schema.org/WebPage" itemid="https://google.com/article"/>

When checking it with the Nu Html Checker, I get this error:

Element meta is missing required attribute content.

Adding an empty content attribute seems to solve this error. Is it correct to do this?