Fixes Issue #1.
A private method triggered the error, so no API changes were necessary.
Caused by this update:
Before 0.26.0, instance variables were allowed, but there were some corner issues regarding typing, semantics, and dependencies. So, from 0.26.0 instance variables are not allowed in the expression that determines the initial values of them.
~ https://crystal-lang.org/2018/08/09/crystal-0.26.0-released.html