Variable access with Map-type objects in Sugarcube 2

I am having trouble with variables of Map type, e.g.,

<<set $map to new Map([['key', "value"]])>>
<<set $map['bla'] to "BLA" >>
<<print $map['bla']>>, <<print $map['key']>>, <<print $map.get('bla')>>, <<print $map.get('key')>>

results in the following output:
BLA, , , value
when I would really expect “BLA, value, BLA, value”.

What is happening here?

Twine Version: xxx (Tweego)
Story Format: Sugarcube 2

So according to this Map and Set, I am probably using the Javascript in the wrong way - one should always use set() and get()… my bad