Legal grammar question - how can this phrase be correct?

[luckyjimm]

I got in an argument with one of the lawyers at the law firm where I'm a lackey about a phrase he wanted me to use in a document:

"an investors' consent"

He insists it is correct. It means one investor, out of a group of investors, is giving his consent.

Similarly, you might have "a dogs' dinner".

To me it just seems wrong; a usefully-short but grammatically impossible formulation.

"The investors' consent" would mean they all consented;

"An investor's consent" would mean there was one investor, and he consented.

I understand the need for a short phrase which means one investor out of a group is consenting.

But you can't have "an investors" or claim that there is no singular/plural mismatch because the "an" connects to the "consent".

Sorry for boring post but this is bugging the hell out of me. Could one of you please explain why this phrase is correct?