Can't use Katarina's shunpo while ulted by Mordekaiser

Got ulted by Mordekaiser, tried to shunpo behind him to dodge his Q but my E wouldn't go off. I spent the rest of the time in the ult testing to see if my e was working, which it wasn't so I died. Come my next life, I get flash ulted by Mordekaiser, dropped a dagger in the death realm and tried to shunpo to the dagger, and I couldn't shunpo to that OR Mordekaiser again and died. I only got ulted by him these two times and both times when he clicked R my Q was in travel to him. I don't know why that would cause my E to not work, I'm just reaching for similarities that would have caused a bug where I couldn't use my shunpo in the death realm.