Issue with keyof

[vivdesign]

Hello,
first of all thanks for the great library.

Now I'm having an issue I don't understand I have class that has a property called fn, this property is an object of functions I need to generate an enum from in my type so I do this
type myFunctions = keyof myClass['fn']
but I get this error:
LogicError: Invalid index "fn" in type "class-1549443271-162-586-1549443271-0-587"
at (node_modules\ts-json-schema-generator\src\NodeParser\IndexedAccessTypeNodeParser.ts:81:27)
at Array.map ()

Could anyone assist I don't understand what I'm doing wrong?