e.findIdx is not a function

hey uh is there any known things that can cause the following error message?

Simulator crashed, no error handler TypeError: e.findIdx is not a function
at Object.s (https://pxt.azureedge.net/blob/4b21e7286becc7c46be95397d4a314bccec49b31/pxtsim.js:1:49755)

ok so i actually fixed it
so i had an array
let arr:[number,number][];

let x = arr[0];

and when i try to access
x[i]

it throws the error

but if i declare x as number[] then it doesnt happen