Calling External API in botkit

Question

As I am new to botkit, I have created the bot locally as per the docs. But when it is trying to call an external API using axios, below error shows -

UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'get' of undefined

Here is the code -

const { BotkitConversation } = require("botkit");
const { axios } = require("axios");

module.exports = function(controller) {
    const convo = new BotkitConversation('convo', controller);

    convo.before('default', async(convo, bot) => {
        axios.get(...https url...) //error line
        .then(function (response) {
          convo.setVar('api-response', response);
        })
        .catch(function (error) {
          console.log(error);
        })
    });

    convo.ask('What is your name?', async(response, convo, bot) => {
        console.log(`user name is ${ response }`);
      }, 'name');
    convo.addAction('msg');
    convo.addMessage('Hi {{vars.name}}! Welcome to my Botkit', 'msg');

}

Can someone point out what is wrong on this code?