<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def pwm_min_score(self):
 """Return the minimum PWM score. Returns ------- score : float Minimum PWM score. """

        if self.min_score is None:
            score = 0
            for row in self.pwm:
                score += log(min(row) / 0.25 + 0.01)
            self.min_score = score
        
        return self.min_score