hi guest, this is the correct behavior of the node. the error your are seeing is due to an index out of bounds exception.
there are many nodes that can fail if you input arguments that the operation cannot process. this is a general thing in vl and we will probably provide language features in the future that help you to handle the exceptions.
for now, if you want to have a fail safe version of it, just wrap it into a new operation that checks the string length and does a modulo on the index accordingly.