ERROR: Message 'at' not understood.

If you have the error Message 'at' not understood then most often this means something is being treated as an ArrayedCollection when it isn't one - for example if it's nil.

( var x; x[4] )

In this simple example, we trigger this error because we haven't initialised x. So x is nil, and nil doesn't know what to do with an at message (remember that x[4] is a shorthand for x.at(4)).

For more info see the helpfiles "Array" and "ArrayedCollection" and "Nil".