Cannot read properties of undefined (reading 'firstName')

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Hello, i write this to code to display on the website the balance of the current memeber connected. So i write this but i get this error message "Cannot read properties of undefined (reading 'firstName')" line 13 i don't know why moreover it's the item of wix-member so i don't understand how it can be undefined. My collection name is "DataBase" and my item for the balance is nammed "solde" and the textblock is "message". I'm beginner btw, be patient with me :) 
Thanks !

const options = {
  fieldsets: ['full']
};

import { currentMember } from 'wix-members';
import wixData from 'wix-data';

currentMember.getMember(options)
  .then((member) => {
    const id = member._id;
    const fullName = `${member.contactDetails.firstName} ${member.contactDetails.lastName}`;
    return member;
  })
  .then((member) => {
    return wixData.get("DataBase", member._id);
  })
  .then((item) => {
    const balance = item.solde; 
    const messageTextBox = $w('#message'); 
    messageTextBox.text = `Solde actuel : ${balance}`;
  })
  .catch((error) => {
    console.error(error);
  });