Failed to decode output: Error

Environment: remix
```solidity
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?

Reproduce the failure in Remix using the contract and inputs shown in the issue. No source file, test, or compiler entry point is identified, so trace the ABI output decoding path to determine why accessing index 0 triggers the deferred error. Done means the cause is established and a focused regression test or fix is identified.

Written by the indexing model from the issue text.