NaN pixels in interferogram


I’ve stumbled upon a problem while following S1 SLC DEM generation tutorial found here:

Even though I am using the same datasets as in tutorial, my interferogram seems to have a lot of “NaN” pixels, which cannot be seen on screenshots in the said article. It kind of looks like pepper spilled all throughout the image. I also tried different datasets, and the issue seems to persist. Any ideas on why is that?