Numbered bibliography entries count up to zero...

I am adding bibliography entries from my bib. database to a writer document.
This worked fine until entry No. 9. Entry 10 was simply listed as 0 in the bib. index, however entry 11 and 12 were then listed as 10 and 11 respectively.

What could be the cause of this? I manually updated all entries from first to last just to be sure there isn’t any issue, but that obviously didn’t fix anything.

The bib. index is set to: number entries - yes; language - English (UK); key: alphanumeric

All my entries are set to “Sort by document position”.