Nullifying Orb not working?

So Just was in a game where I had Null Orb vs a Kat mid. Block zero damage. I think once the buff icon was gone the first time it never came back. Tool tip says at the end it was to block 171 damage but had blocked 0. For this Rune to be worth picking it needs to have its CD scale with Level down to about 12s at lvl 18
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