Help with installing Portal content into GMod dedicated server?

I’ve followed the Gmod wiki entry on setting up a dedicated server almost to the letter, but now I’m getting an odd error I don’t know the cause of.

After installing portal onto srcds (/orangebox/portal/), every time I try to spawn a portal prop in-game the server crashes, with utlmemory.h (322) throwing a debug assert for IsIdxValid(i). No other prop or content causes this, just Portal. Is there any particular reason this would be happening?

Bump with a solution, or at least what I think was the solution. In following the guide “almost” to-the-letter, I’d changed it by installing it into C:\Program Files\Valve\HLServer. This seems to have been the problem; reinstalling the server in C:\srcds\ has fixed the problem.