Replacing Nth element of array

[jeremy.rutman]

I can replace the element at a fixed index of an array using the Assign of Structured Element Array Operator. 
However I need to replace the Nth element, where N is an input to my function - when I try this (using N instead of some constant) I hit 

Type Error ERR_105 Not a size expression at REFRESH_CMD_TIMEOUT/_L21= 

Identifier N should be bound to a static size expression 

Can someone let me know if a. this is possible, and b. if possible, how to proceed

[jeremy.rutman]

For the Assign of Structured Element hidden parameter (the index) you can set [N] in  the parameters box at the bottom. 
Why this allows you to  now use dynamic index is a mystery to me.  If you go out of the array bounds nothing happens, so apparently Scade knows not to write past array bounds. 
But if it knows how to do that then why throw the 'Identifier N should be static' that started me on this little journey?