PortalSiteMapProvider was unable to fetch children for node at URL: message: Object reference not set to an instance of an object

I was seeing this error the other day on our test publishing site, and tracked it down to an issue in the web.config file.  Seems like our web.config was using the wrong default provider:

<siteMap defaultProvider="CurrentNavSiteMapProvider" enabled="true">

where it should have been to set to:

<siteMap defaultProvider="CurrentNavigation" enabled="true">