InvalidParameterType: Expected params.Username to be a string

forgotPassword(username: string){
       console.log("forgotPassword called for..", username);

       let appClientInfo = {
                        "userpoolId": "pool ID",
                        "appClientId": "client ID"
                        }

       var poolData = {
            UserPoolId : appClientInfo.userpoolId, // Your user pool id here
            ClientId : appClientInfo.appClientId // Your client id here
        };

        var userPool = new AWSCognito.CognitoIdentityServiceProvider.CognitoUserPool(poolData);
        console.log("userPool..",userPool);
        var userData = {
            Username : username,
            Pool : userPool
        };

        var cognitoUser = new AWSCognito.CognitoIdentityServiceProvider.CognitoUser(userData);
        console.log("cognitoUser..",cognitoUser);

        cognitoUser.forgotPassword({
        onSuccess: function (result) {
        // successfully initiated forgot password request
        console.log('CodeDeliveryData from forgotPassword: ' + result);
        this.router.navigate(['login']);

        },
        onFailure: function(err) {
            alert(err);
            console.log('error: ' + err);
        }

    });
   }