Why is unittest returning ValueError (not in list) when the element is in the list?

I have a function which searches a Bank for a specific Account object. If it finds a matching account, the function replaces the Account object with None. If it finds no match, it returns. This works perfectly in IDLE.

My unittest for this function keeps returning ValueError: <Mock id='2350965017280'> is not in list.

Here is the function:

def removeAccountFromBank(self, account):
        target_index = accounts.index(account)
        accounts[target_index] = None
        return

Here is my test (using unittest):

def test_removeAccountFromBank(self):
        account = Mock()
        bank = Bank()
        accounts = [None, (account), None]
        print(accounts)
        bank.removeAccountFromBank(account)
        self.assertEqual(bank.get_accounts(), [None, None, None])

I expected this test to pass, but here is the result I got:

The print statement correctly shows the account: [None, <Mock id='2350965017280'>, None]

But I get an error when the function tries to find the account using index:

in removeAccountFromBank target_index = accounts.index(account) ValueError: <Mock id='2350965017280'> is not in list

I am confused about why the accounts.index part is returning a ValueError. I also tried printing the accounts.index(account) and it returns the correct index of 1.