Can a Californian employer dock your pay for mistakes that did not cost them anything?

Sometimes I make some typos at work but I always end up correcting them and the company has not lost any money, nor have any customers ever gotten upset/complained. My employer did not make me sign any contract saying that I agreed to this but today she told me that if I make any more mistakes that she will dock my pay. However, I am wondering if this is legal. If so, how much pay should be deducted for a typo where no harm was done? Can anyone advise me on this issue? Thank you.