function inside a function project online Odata url query

Question

I am trying to create a Project Online OData URL where I use two functions, one inside the other $filter=substring(ResourceName, indexof(ResourceName,','),10) eq 'example'

substring is the first function and index of is the second function called inside the substring brackets and it returns an integer.

so, I get this message back:

Fehler beim Verarbeiten dieser Anforderung.

and I just want to know why and whether I can do the same some other way