Order of operations in an assign shape

I keep forgetting this, but when you create a message in an assign shape, you have to assign to all the message parts before you can assign values to any message context property.

trying to do something like -

MyMessage.PartA = something;
MyMessage(SomeContextProeprty) = someValue;
MyMessage.PartB = somethingElse;

Will result in an error about accessing an unconstructed message.
changing the code to -

MyMessage.PartA = something;
MyMessage.PartB = somethingElse;
MyMessage(SomeContextProeprty) = someValue;

compiles with no problem