You may be able to fix this by the following:
1. Deactivate and delete any installed XML sitemap-building plugin (other than the Yoast plugin).
2. Deactivate the Yoast plugin.
3. Delete existing XML sitemap files, both normal and GZIPed.
4. Reactivate the Yoast plugin.
5. Use the Yoast plugin to create the XML sitemap.
The sitemap should now create normally and the error should go away.
I initially had this same problem because the Google XML sitemaps plugin was installed. Even deactivating it did not work. Following the procedure above corrected the problem.