Undefined property: stdClass::$book in eval()

Here is the background: I have Custom Breadcrumbs Alpha1 running on a D7 install. I have a "Page" content type on the site. Some of the pages are part of a Book some are not. I have setup breadcrumbs for both page types.

The breadcrumbs for the Book Pages use a custom PHP snippet: if (is_array($node->book)) { return true; }

The non-book pages use the standard custom breadcrumb.

The breadcrumbs for both page types are working (outputting the correct link names and paths), but, when I am on a non-book Page node I get the following error message:

Notice: Undefined property: stdClass::$book in eval() (line 1 of site-url.../modules/contrib/custom_breadcrumbs/custom_breadcrumbs.module(125) : eval()'d code).

Any idea what this could be?