Scaling up a math symbol messes up toc

Question

I am trying to scale up a greek letter inside a math formula. To do this, i follow this answer and use \scaleobj from the package scalerel

However, when I try to use an expression that includes my upscaled symbol in a section heading and use \tableofcontents, errors occur.

What is the reason for the errors? How can I use my little formula in a section heading without breaking my table of contents?

Here is a mimium error producing example:

% minimum working example to demonstrate a problem with math mode and scalebox not working together with headers and table of contents
% this is the base class of my document
\documentclass{scrbook}
% minimum number of imports
\usepackage{scalerel}
\usepackage{amsmath}
% define upscaled \pi
\newcommand{\PI}{\scaleobj{1.5}{\pi}}
% use this symbol inside math mode
\newcommand{\mypi}{(\PI_\text{mypi})}
\begin{document}
\tableofcontents
\chapter{Main Chapter}
Some text about \mypi.
% comment out the following line to get a minimum working example
\section{What's the matter with \mypi?}
Some more text.
\end{document}