Section 0 created out of the blue

Using H2 for chapters and H4 for sections in a page, the TOC shows as

2 – Chapter 2
2.0.1 Section 1
2.0.2 Section 2

rather than

2 – Chapter 2
2.1 Section 1
2.2 Section 2

Looking at the code of the page in the WP editor, there is nothing in between those headings.

Any idea?