is tryndamere meant to be able to ult while he is cc'd

i was playing malzahar and i would ult tryndamere at about half or quarter health hoping to kill him before he could pop undying and yet he would hit zero and bam undying then spin away or just murder me. the trynd even said he was ulting while surpressed. i had always thought trynd had to activate his ult before he got cc'd and that there was counter play to his ult. so the question is is this a glitch or does trynd just really not care about cc?