Cas Mutate allows Decrement/Increment with invalid Cas value

var key = "foo" + DateTime.Now.Ticks;
var decrementResult = client.Decrement(key, 100, 10);
var casResult = client.GetWithCas(key);
var result = client.Decrement(key, 100, 10, cas.Cas - 1);

The Decrement/Increment operation will succeed, even though the Cas value was not valid for that key.