web3js 1.0 fromWei throwing although passing correct type

Question

I have encountered a problem with the web3js 1.0 library.

I have a simple function that is getting a balance from a smart contract:

async balance() {
  return await this.Marketplace.balances(this.storeOwner);
}

I have another function inside my angular component where I display the balance to the user:

async getBalance() {
  const balance = await this.smartContract.balance();
  console.log(balance);
  this.balance = this.smartContract.web3.utils.fromWei(balance, 'ether');
}

When calling the web3.utils.fromWei() function I get this error:

Error: Uncaught (in promise): Error: Please pass numbers as strings or BigNumber objects to avoid precision errors.

but the console log shows me, that the balance is a BigNumber object:

BigNumber {s: 1, e: 0, c: Array(1)}

I am using version 1.0.0-beta.36. Why do I get the error?