Probably the same as when she did the same test with children or explicitly asked for "nonconsensual nude" - the system refused to generate pornographic images because of safeguards.

  Weatherbed noted that asking Grok directly to generate non-consensual nude Swift images did not generate offensive outputs, but instead blank boxes. Grok also seemingly won't accept prompts to alter Swift's appearance in other ways, like making her appear to be overweight. And when Weatherbed tested using "spicy" mode on images of children, for example, Grok refused to depict kids inappropriately.