What is non-existent cannot ever be produced. Whatever is non-existent remains non-existent for ever and whatever is existent always exists. Nothing can be sometimes existent and sometimes non-existent. Self, for example, is always existent, whereas the fictitious sky-flower is eternally non-existent. No agency can turn non-existent into existent. So if the effect were non-existent in the material cause before the causal operation, then it would never be produced.