Modifying state of array element in Harlowe 2.0.11

So I've declared

(set: $foo to (a: false, false, false))

For some reason, I can

(set: $foo's 1st to true)

but I can't

(set: $foo[0] to true)

The latter keeps saying unexpected identifier. I'm trying to do something kind of complicated but in order to isolate the problem I wrote this simple code:

(set: $foo's 1st to true)
(if: $foo's 1st is true)[bar]

That works. This doesn't??

(set: $foo[0] to true)
(if: $foo[0] is true)[bar]

I want to use a variable in the $foo array so I don't have to repeat code. Am I doing something wrong...?