Hardhat 'function returned an unexpected amount of data' means you called a contract method on an EOA

Symptom, from a Hardhat test or a view call through ethers:

Error: Transaction reverted: function returned an unexpected amount of data at Institution.getInstituteData (contracts/Institution.sol:112)

The named line does not obviously do anything wrong, and it is a view function, so there is no state change to blame.

Cause: somewhere on that line the contract calls a function on an address that holds no code — typically SomeContract(msg.sender).someFn() where msg.sender is a test EOA rather than the contract you assumed. The EVM does not revert when you call an address with no code; the call returns success with zero bytes of return data. Solidity then tries to ABI-decode a value out of nothing, and the decode failure surfaces as this message. In the reported case the offending call was Certification(msg.sender).owner() while the test invoked the function directly from a Hardhat signer.

Fix: make the caller actually be the contract whose interface you cast to. Add a forwarding function on that contract and call through it, rather than trying to spoof msg.sender from the test:

function getInstituteData(address institutionAddress) external view returns (...) { (instituteName, instituteAcronym, instituteLink, instituteCourses) = institution.getInstituteData(institutionAddress); }

An ethers { from: contractAddress } override does not work — from selects a signer, and a contract has no key to sign with.

Generalise it: this message almost always means "cast to an interface, called an address with no code (or with a different ABI)". Check the target with eth_getCode before debugging your decoding. It also appears when the address is right but the function signature does not exist there, giving the same empty returndata.

Source: adapted from a Stack Exchange answer by Mila A (question by Nikhil Raj), https://ethereum.stackexchange.com/questions/160049/transaction-reverted-function-returned-an-unexpected-amount-of-data-when-reques — CC BY-SA 4.0