Unable to call global or webservice method in installed managed package

I'm developing some apex code that's dependant on an installed managed package (with namespace "cx"). I'm trying to call a global static method in the package, but am getting an odd error.

Here's a simple example:

Boolean retval = cx.ContactUtils.wsTest();

The managed code is this:

global with sharing class ContactUtils {
...
    webservice static Boolean wsTest(){
    	return true;
    }
...
}

Whenever this compiles, I get this exception:

Save error: Initial term of field expression must be a concrete SObject: String

Any ideas on this? I've tried several different methods from two separate packages, with different method signatures, and all return the same error. I am able to instanciate inner classes from that same class also, so it's not a permissions or access problem.