Find the flaw in this roulette strategy

I am not much of a roulette player but I found a fresh link with an (in)famous roulette strategy that goes like this.

“Go to European Roulette on any casino site. Try Rushmore Casino or Unibet for example.
You start by betting 1 euro on black and spin the wheel. If the ball drops onto red, you bet 2 euro on black. If the ball drops on red again, you bet 4 euro on black. You keep betting on black until you win. As soon as you have won on black you start betting 1 euro again, but this time on red. The key is to keep doubling up if you don’t win.”

I would like to challenge you in coming up with the quite obvious flaw to this roulette strategy. And if you try it, remember that your were warned here.

Good luck at the tables!