OCMock 3 not matching the string argument

I'm starting to use OCMock (3.1.2) in my project for unit tests, and I'm having a weird behaviour. See the following test:

- (void)testOCMock
{
    OCMExpect([self.delegate openEmailComposer:@"me@email.com"]);

    [self.delegate openEmailComposer:@"hello there"];
}

Surprisingly, the test succeeds. Am I missing something?