Word 365 When I create a custom table of contents the table includes a style that is not in document. The document is using my custom styles: Heading 1, Heading 2, Heading 3 and Heading 4. And I want to create the table of contents with these styles.

However the Table Of contents tab lists these styles and one more: Title1. This style is not in the current document. But in the Table of Contents options, Title1 is assigned to TOC level 3. I have to delete the 3 for to remove it from the TOC. If I use a built in TOC (not custom) Title1 does not appear in the table of contents.

I have to remove Title1 every time I create a custom table of contents.
What could be causing this and how do I prevent it from happening?