Failed to decode output: Error

[Subway2023]

Environment: remix

contract C{
    function k(string calldata a, bytes memory b) public returns (string memory) {
        return string.concat(a, string(b));
    }
}

input: 0x40, 0x80
output: Failed to decode output: Error: deferred error during ABI decoding triggered accessing index 0

Why does the decoding error occur?