Perforce Configuration Issue

I've encountered a problem with my project's Perforce configuration in 122.264. I use a single client spec for Perforce. Root is set to c:\. The project I'm working on is in a subdirectory, c:\dev\moo\project. This has always worked fine with IDEA in the past. Now, when my project is loaded, IDEA complains that Perforce is offline. When I use Test Connection, I see the following error:

Wrong client specification:

Client roots: c:\

Actual root: C:/dev/moo/project

(The mixture of forward- and backslash is present in the actual error message text.) This happens whether I use P4CONFIG or supply the connection parameters in IDEA.