Is there a cast time on malzahar's ult or something?

I was playing malz and was laning against an akali at least 4 times during that game, when akali ulted me, i tried to ult her while she was still mid way to me, but my ult didn't activate and consequently i died over and over due to the ult not activating when i needed it to. It still cast fine when there was plenty of time for me to cast, but when i was trying to burst her, it wouldn't work or at least wouldn't cast immediately