IndexErrorRaised by an out-of-bounds index (e.g. List.at:put:, Bytes.at:). The VM populates index (the offending index) and length (the collection's size) so a handler can report or recover without parsing the message.
The out-of-range index the operation was asked for.
{ #(1 2).at:9 put:0 }.catch:{ |e:IndexError| e.index } "* -> 9
The receiver's length at the time of the failure.