Poisson Intervals with No Events

Events arrive as a Poisson process with unknown rate λ per minute. A logger is too crude to count events: for each of 50 independent one-minute intervals it records only whether the interval contained no events. Exactly 20 of the 50 intervals were empty. What is the maximum likelihood estimate of λ? Give an exact closed form (a natural logarithm), or a decimal rounded to 3 or 4 places.