Incorrectly accessing mapped array?

Question

I have:

mapping (uint256 => uint64[]) public Arry;

I am trying to do:

Arry[123][0]++;

And I am getting the generic invalid opcode.

To me, I am accessing map key 123 which is a uint64 array. I am then incrementing the uint64 in position 0 by one. I don't understand why it dies here.

Answer 1

Arry[123][0] is out of bounds if Arry[123] has a length of 0. Make sure the array has at least one element in it.

Your code works fine for me as long as I do Arry[123].push(0) somewhere first.