Atm help

Hey just an error that I cant fix nor can I find anything on the web to help me solve this.
ATM1:12:attempt to compare nil with number

term.clear()
local balance = 100
repeat
  term.setCursorPos(1,1)
  term.setTextColor(colors.red)
  term.write("====sflynn's User Friendly Banking balance = "..balance.."====")
  term.setTextColor(colors.blue)
  term.setCursorPos(1,2)
  term.write("how much would you like to withdraw: ")
  local withdraw = tonumber(read())
  while withdraw > balance and balance > 0 do
	balance = balance - withdraw --balance = balance [b]minus[/b] withdraw
  end

I think I see the problem. When you run through it and withdraw x dollars, it works. but there is nothing to clear the amount that you entered before. So when you press enter the second time, the value of withdraw is nil.

EDIT :

I'd suggest adding the term.clear() into the loop.
Like so:

local balance = 100
repeat
  term.clear()
  term.setCursorPos(1,1)
  term.setTextColor(colors.red)
  term.write("====sflynn's User Friendly Banking balance = "..balance.."====")
  term.setTextColor(colors.blue)
  term.setCursorPos(1,2)
  term.write("how much would you like to withdraw: ")
  local withdraw = tonumber(read())
  while withdraw > balance and balance > 0 do
	    balance = balance - withdraw --balance = balance [b]minus[/b] withdraw
  end
until nil

If anything other then a number is entered (eg a $ symbol), tonumber(read()) will return nil.

thanks!