Nimscript: setLen segfaults?

[Udiknedormin]

When I used keepItIf in a nimble scirpt, it segfaulted. After copy-pasting the 
templates body and trying to play with it a little bit it turned out setLen was 
the one to blame. If I replace moving the elements to adding them to another 
seq and then assign it to the original one, everything works fine. Does anyone 
know why is that so or how can I prevent it from segfaulting?