Mock of UserManager again

I have the following code:

        var userStore = new Mock<IUserStore<AspNetUser, int>>();
        var userManager = new ApplicationUserManager(userStore.Object);
        controller.UserManager = userManager;

        userStore.Setup(x => x.CreateAsync(It.IsAny<PixlocateDB.AspNetUser>())).Returns(Task.FromResult(IdentityResult.Success));

It works fine. Now I want to create a stub for AddToRoleAsync. I try to do the following:

        userStore.Setup(x => x.AddToRoleAsync(It.IsAny<int>(), It.IsAny<string>())).Returns(Task.FromResult(IdentityResult.Success));

but I get an error:

Severity Code Description Project File Line Suppression State Error CS1061 'IUserStore' does not contain a definition for 'AddToRoleAsync' and no extension method 'AddToRoleAsync' accepting a first argument of type 'IUserStore' could be found (are you missing a using directive or an assembly reference?) Pixlocate.Tests

Why?