AWS Amplify/Lobby

Hello, I am stuck with the below error when using aws-amplify in my react app. Auth.signIn is returning the confirmed user from cognito pool. I also receive the verification code via SMS. However, when I try to call Auth.sendCustomChallengeAnswer I am getting below error. Note that Auth.signIn and Auth.sendCustomChallengeAnswer are in two different methods. Please advise.

code
`
handleSignIn = (a) => (param) => (e) => {
if (this.validator.fieldValid('First Name')
&& this.validator.fieldValid('Last Name')
&& this.validator.fieldValid('Phone Number')) {

  let username = "+01" + this.state.phoneNumber.replace(/[^\d]/g, '');
  Auth.signIn({
      username, // Required, the username
  }).then(user => {
    console.log(user);
    this.setState({
      ['formActivePanel' + a]: param,
      ['formActivePanel' + a + 'Changed']: true,
      cognitoUser: user,
    });
  })
  .catch(err => {
    console.log(err);
    //TODO Handle ERROR
  });
} else {
  this.validator.showMessageFor('First Name');
  this.validator.showMessageFor('Last Name');
  this.validator.showMessageFor('Phone Number');
}

}

handleVerify = () => {
if (this.validator.fieldValid('Verification code')) {
let challengeResponse = "123456";//this.state.otpCode;
let user = this.state.cognitoUser;
console.log("otpCode:" + challengeResponse);
console.log(this.state.cognitoUser);

  Auth.sendCustomChallengeAnswer({
    user,
    challengeResponse,
  }).then(user => {
    console.log(user);
  })
  .catch(err => {
    console.log(err);
    //TODO Handle ERROR
  });

} else {
  this.validator.showMessageFor('Verification code');
}

}
`

ERROR
[ERROR] 57:50.242 AuthError - Challenge response cannot be empty