i cant seem to figure out the error

[anthony uwaifo]

hi everyone,

please i need help with this assignment. I have written a code and i still
get an error. please help me debug my code.

instructions:

   - Create a constructor that takes in an integer and assigns this to a
   `balance` property.
   - Create a method called `deposit` that takes in cash deposit amount and
   updates the balance accordingly.
   - Create a method called `withdraw` that takes in cash withdrawal amount
   and updates the balance accordingly. if amount is greater than balance
   return `"invalid transaction"`
   - Create a subclass MinimumBalanceAccount of the BankAccount class


My code:

class BankAccount(object):
  def __init__(self, balance):
    self.balance = balance


  def deposit(self, amount):
    self.amount=amount
    self.balance += amount
    return self.balance


  def withdraw(self, amount):
    self.amount=amount
    if(amount > self.balance):
      return ("Amount greater than available balance.")
    else:
  self.balance -= amount
    return self.balance



class MinimumBalanceAccount(BankAccount):
  def __init__(self, minimum_balance):
    BankAccount.__init__(self)
    self.minimum_balance = minimum_balance

act = BankAccount(5)
act.deposit(400)
act.withdraw(200)
print act.balance


error message:

THERE IS AN ERROR/BUG IN YOUR CODE*Results: *
{"finished": true, "success": [{"fullName": "test_balance",
"passedSpecNumber": 1}, {"fullName": "test_deposit",
"passedSpecNumber": 2}, {"fullName": "test_sub_class",
"passedSpecNumber": 3}, {"fullName": "test_withdraw",
"passedSpecNumber": 4}], "passed": false, "started": true, "failures":
[{"failedSpecNumber": 1, "fullName": "test_invalid_operation",
"failedExpectations": [{"message": "Failure in line 23, in
test_invalid_operation\n
self.assertEqual(self.my_account.withdraw(1000), \"invalid
transaction\", msg='Invalid transaction')\nAssertionError: Invalid
transaction\n"}]}], "specs": {"count": 5, "pendingCount": 0, "time":
"0.000079"}}
205