Pliny 2.11.1 - Genitive PPP?

Could anyone help me with the first sentence of one of Pliny’s Letters (2.11.1):

Solet esse gaudio tibi, si quid acti est in senatu dignum ordine illo.

I understand the sense (“You are always happy whenever something happens in the senate that becomes the rank of its members.”) but am stumped by acti.

As far as i can make out, I think it is a genitive sg. masc./nom. of the past perfect participle of ago (agoactumacti), governed by quid. I’m used to clauses of si quid + the genitive of an adjective or substantive, but don’t really get how it can take a PPP like this. Nor can I find the correct passage in G&L that might help me. Then again, I might have parsed it incorrectly…

Any help?

It’s just the genitive of actum, often used as a noun.

Ok - it’s staring me in the face! Thanks