How to partially mock external object

I have class method to test with dependant object (Keys object)

+ (NSURL*)apiURLWithPath:(NSString*)path {
    MyKeys *keys = [MyKeys new];
    NSString *url = [NSString stringWithFormat:@"%@?api_key=%@", path, [keys APIKey]];
    return [NSURL URLWithString:url];
}

I am trying to partially mock this Keys object and return "MY_API_KEY" value but the test method fails and returns real API key (e.g. as78d687as6d7das8da).

describe(@"APIRouter", ^{
    it(@"should return url for api", ^{
        Keys *keys = [Keys new];
        id keysPartialMock = OCMPartialMock(keys);
        OCMStub([keysPartialMock APIKey]).andReturn(@"MY_API_KEY");
        NSURL *url = [APIRouter apiURLWithPath:@"http://ift.tt/1Q4Z61E"];
        expect([url absoluteString]).to.equal([NSString stringWithFormat:@"http://ift.tt/1Q4Z3TI"]);
    });
});