VM error: invalid opcode error caused by a modifier

Question

The following code allows a user(defined by userid) to subscribe to a policy (defined by policy address of the policy smart contract)

function subscribeToPolicy(address policyAddress, uint userId) public payable {
    uint policyPrice = 0.1 ether;
    require (msg.value >= policyPrice);
    userMapping[userId].mySubscriptions.push(policyAddress);
}

This piece of code works as expected (after supplying 0.1ETH and appropriate gas) and the policy is added to the user's subscription array.

I modified this code to add a modifier to prevent the addition of duplicate policies as follows

modifier policyExists(address policyAddress, uint userId) {
    for(uint j=0; j<userMapping[userId].mySubscriptions.length+1; j++){
        if (userMapping[userId].mySubscriptions[j] == policyAddress ){
            emit Log("Policy already purchased");
            revert();
        }
    }
    _;
}
function subscribeToPolicy(address policyAddress, uint userId)            
policyExists(policyAddress, userId) public payable {
    uint policyPrice = 0.1 ether;
    require (msg.value >= policyPrice);
    userMapping[userId].mySubscriptions.push(policyAddress);
}

This code does not work as expected and the error returned by the VM is as follows

transact to OasisUser.subscribeToPolicy errored: VM error: invalid opcode. invalid opcode

The execution might have thrown. Any ideas for solving this error?

Answer 1

You are attempting to access the array out of bound.

Change this:

j<userMapping[userId].mySubscriptions.length+1

To this:

j<userMapping[userId].mySubscriptions.length

BTW, I might be wrong here, but when a transaction reverts, nothing is written into the blockchain. So emitting an event beforehand (emit Log("Policy already purchased")) is useless, because it is not going to be registered.