Recently I start culturing purple sulfur bacteria, but when I prepare the medium, black precipitant formed, I guess it is FeS, is this normal? or I did something wrong?

I use the recipe from "C. B. VAN NIEL, Techniques for the Enrichment, Isolation, and Maintenance of the Photosynthetic Bacteria", which have NaS and Fe(II)SO4 that I think cause the precipitant.