A PHP Error was encountered / Message: Undefined index: 10

I have received this error:

A PHP Error was encountered

Severity: Notice

Message: Undefined index: 10

Filename: relationship_parser/Tree_builder.php

Line Number: 536

I get 6 of these.

I have figured out what section of code is the problem. It is a relationship within a relationship. To limit amount of editing and allow the client multiple options, had to do it that way. Have other relationships within relationships elsewhere on the site and no errors. The code still works though, it displays/outputs correctly. Does anyone know what this error is or how to go about debugging it?