wallet.getBalance() is unreliable in production

[ghost]

I'm using the following code on both the local machine (dev server) and on the remote server (AWS Lightsail). When I'm developing locally everything works perfectly, but when the code runs on the remote server getBalance method ocasionally returns 0 (~3 out of 10 requests).

I added loggers to getResult and perform functions in json-rpc-provider.
Input to perform never changes (address and blocktag are always the same)
When a promise resolves to 0, output of getResult is Object {jsonrpc: "2.0", id: 42, result: "0x0"}

const testnet = true
const providerToken = '...'
const privateKey = '...'

const provider = new ethers.providers.InfuraProvider(testnet, providerToken)
const wallet = new ethers.Wallet(privateKey, provider )

wallet.getBalance()
  .then(balance => { console.log(balance.toString() }) // logs 0 ocasionally (3 out of 10 requests)

Any idea what is going on?