Matlab internal error problem

For some reason the attached nested function file produces a MATLAB system error, even though I do not even execute it. If I open the file and scroll down to the bottom and change the variable "a" to anything else and save the file I get "Matlab has encountered an internal problem and needs to close". Sometimes I do not even have to save the file, just leave it opened and Matlab will crash after a while.

I tried this on two different computers (one Windows 10, one Windows Server 2012 r2) with two different versions of Matlab (R2017b and R2016b). I also rewrote the complete function, resulting in the same error.

Any ideas?

Edit: You can also crash Matlab by setting a Breakpoint! I also tried Matlab R2014a... Same result.