Why DVB-S2 Demo shows BER=0 when SNR(dB)=0?

There is a task to get diagrams of BER on the signal / noise ratio. I used already written a demo program from MATLAB, which models the channel, modulation and coding LDPC code of different speeds. I have some questions. For each value of the signal / noise a few dozen times, I send messages, then the total number of bit errors from all reports divide by the total number of bits in all messages. So I get a Bit Error Rate. Is it right way to get Bit Error Rate?

For speeds where code redundancy is large enough (speed 2/3 and below) for a signal / noise ratio = 0 BER is also equal to 0. Why is this so?