It likely depends what you are doing before that code. You would be best to pre-size your array of objects with e.g.
Bolt(1,1).PipeBolt = PipeLine.empty(N,0);
to ensure that the property is set to be of the type 'PipeLine', otherwise it will be a double by default, like all variables, so if you didn't explicitly instantiate it and then try to set e.g. its 7th element it will have created it as a double and be expecting a double to be given to it, not some other object type.
In general I initialise all my properties that are to be some other object as e.g.
myProp = SomeClass.empty;
or something like the example above if I expect it to be an array of objects. I cannot remember off the top of my head though - just doing the above and telling it the type of object without presizing may well be enough that your code causing an error would work because it knows the object type beforehand.
