Confluence throws Can't find brush for language

Steps to reproduce:

1. install Confluence 5.9.x
2. Configure Code Macro via Administrations > Configure Code Macro > add a new language.
   • eg: add Luo highligher as per https://drakeor.com/wp-content/uploads/2015/01/shBrushLua.js
3. create a new page
   1. insert Code Macro
   2. select the newly created syntax highlighter

Expected Behaviour:
Code will be highlighted nicely

Actual Behaviour:
Error "Can't find brush for: <language>" is thrown when the page is accessed

Misc:
The same Custom Brush works in Confluence 5.7.x and Confluence 5.8.x