It seems like there was a “minor” bug impacting some of the reports for the Sitemaps indexing count.

A Google Webmaster Help thread had a couple of webmasters come in asking why Google was reporting that all of a sudden, the index count was down to zero.

As you can imagine, having an index count of zero can be very stressful even if you are still getting traffic from Google.

Many people rely on Google Webmaster Tools for the reporting and to determine the health of a site in Google’s index.

Google confirmed their was a “minor bug” that has been fixed to address the sitemaps reporting error. Googler, Asaph said:

This comes recently after a minor reporting bug with the link tool. No wonder 68% of SEOs find webmaster tools inaccurate on some level.

Forum discussion at Google Webmaster Help.