AWS Lambda TokenAuthorizer response error - NodeJS

I am creating a JWT token authorizer and I am nearly their I think, I just am getting an error when testing it.

So I created a Lambda token authorizer which will authorize a token on an incoming request to the gateway.

Here is the code:

const blahJWT = require("@custom/blah/authentication/blah");

exports.handler = async (event, context, callback) => {

    try {
        let token = extractTokenFromHeader(event) || '';
        const webAuth = new blahJWT();
        await webAuth.authenticateWebToken(token);
        
        callback(null, 'Allow');


    }
    catch (e) {
        console.log(e);
        // return {body: "Unauthorized", statusCode: 401};
        callback('Unauthorized');
    }
};

function extractTokenFromHeader(e) {
    console.log(JSON.stringify(e));
    console.log(e.authorizationToken);
    if (e.authorizationToken && e.authorizationToken.split(' ')[0] === 'Bearer') {
        return e.authorizationToken.split(' ')[1];
    }
    else {
        return e.authorizationToken;
    }
}

However when I pass in a valid token through the api-gateway tester I get the following error:

Tue Sep 13 23:01:46 UTC 2022 : Authorizer result body before parsing: "Allow"
Tue Sep 13 23:01:46 UTC 2022 : Execution failed due to configuration error: Invalid JSON in response: Cannot construct instance of `com.amazonaws.backplane.executioncore.frontend.authorizer.CustomAuthResponse` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('Allow')
Tue Sep 13 23:01:46 UTC 2022 : AuthorizerConfigurationException

I am wondering if anyone can help solve this?

Thanks