Twig "Is defined" throwing error on matrix row EntryModel property

Question

When checking if a matrix block property is defined, I'm getting an error:

Craft\EntryModel and its behaviors do not have a method or closure named "contentSource".

The line that is throwing this error is specifically checking if that property is defined.

{% if block.contentSource is defined %}
    {# Stuff and things #}
{% endif %}

-- Or --

{% set theThing = block.contentSource ?? "fallback" %}

Am I expecting the "is defined" check to be smarter than it actually is? I can't see an example of this in the matrix field or craft twig/templating docs.

Thanks :)