0.23.1 throws internal error: reached unreachable code exception

While 0.23.0 worked fine, 0.23.1 seems unusable on my end as I'm hit with the same error every time:

thread '' panicked at src\backtrack.rs:300:17:

internal error: entered unreachable code

note: run with RUST_BACKTRACE=1 environment variable to display a backtrace

Wish I could be more helpful than that, but sadly in this case I don't even know what I'm looking at.