How to use vm.load to get mapping value?

[linghuccc]

In my solidity contract, there is a private mapping parameter (address => uint256). I used "forge inspect" and found out that it is stored in Slot 8.

I tried below command to retrieve a mapping value of a given mapping address:
bytes32 amount = vm.load(
contractAddress,
keccak256(abi.encode(mappingAddress, uint256(8)))
);
but the result was 0x0000000000000000000000000000000000000000000000000000000000000000.

Why is this so?