"Error: VM Exception while executing eth_call: invalid opcode" when accessing array

Question

I'm trying to incorporate storage in a smart contract where participants of the contract will have their address stored in a address[] look up table participationAddressLUT.

The contract initializes with 2 addresses in the look up table via the function bootstrapParticipation(). The participationSize() getter gives us the size of the look up table.

However when trying to retrieve the size of the lookup table using web3 v0.20.2

TokenSale.at(this.state.contractAddress).then((instance) => {
    instance.participationSize.call().then((r) => {
        console.log('participationSize: ', r)
    })
})

we get the cryptic error

Uncaught (in promise) Error: Error: VM Exception while executing eth_call: invalid opcode

What is wrong with the current contract code that prevented this from working?

Contract code:

pragma solidity ^0.4.15;
import "./libs/zeppelin/crowdsale/CappedCrowdsale.sol";
import "./libs/zeppelin/crowdsale/FinalizableCrowdsale.sol";

contract TokenSale is CappedCrowdsale, FinalizableCrowdsale {

    mapping (address => uint256) public participationWeis;
    address[] public participationAddressLUT;

    function TokenSale(uint256 _startTime, uint256 _endTime, uint256 _rate, uint256 _goal, uint256 _cap, address _wallet, address _tokenContractAddress)
        CappedCrowdsale(_cap)
        FinalizableCrowdsale()
        Crowdsale(_startTime, _endTime, _rate, _wallet)
    {
        bootstrapParticipation();
    }


     function bootstrapParticipation() internal {
        address participant = 0xcede48d8ac162d1b08ed9419010de3c99f2cfdd6;
        uint256 weiAmount = 1000000;
        participationWeis[participant] = participationWeis[participant].add(weiAmount);
        participationAddressLUT.push(participant);

        participant = 0x854bd635fd4e8684a326664e0698c8fefae6dd97;
        weiAmount = 5000000;
        participationWeis[participant] = participationWeis[participant].add(weiAmount);
        participationAddressLUT.push(participant);
      }


    function participationSize() public returns (uint256) {
        return participationAddressLUT.length;
    }

}

Full Traceback

errors.js:35 Uncaught (in promise) Error: Error: VM Exception while executing eth_call: invalid opcode
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:48863:17
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:60103:5
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:12319:9
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:8879:16
    at replenish (C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:9399:25)
    at iterateeCallback (C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:9389:17)
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:9364:16
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:12316:13
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:60099:9
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:54779:7
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:48863:17
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:60103:5
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:12319:9
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:8879:16
    at replenish (C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:9399:25)
    at iterateeCallback (C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:9389:17)
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:9364:16
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:12316:13
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:60099:9
    at C:\Users\y\AppData\Roaming\npm\node_modules\truffle\build\chain.bundled.js:54779:7
    at Object.InvalidResponse (http://localhost:3000/static/js/bundle.js:94294:17)
    at http://localhost:3000/static/js/bundle.js:90537:37
    at XMLHttpRequest.request.onreadystatechange (http://localhost:3000/static/js/bundle.js:59860:8)

Answer 1

The Solidity function looks fine. Getting the contract instance is not a promise.

Try calling the function from the instance, as follows:

var instance = TokenSale.at(this.state.contractAddress);
instance.participationSize.call().then((r) => {
    console.log('participationSize: ', r);
});