Mocking stored procedure's output parameter

I have following method

public bool IsUserAllowedToDoThings(string userName, string thingToDo)
    {
        var outputParameter = new ObjectParameter("IsAllowed", typeof(bool?));
        _context.SP_IsUserAllowedToDoThings(userName, thingToDo, outputParameter);
        return (bool)outputParameter.Value;
    }

The method just calls SP using EF and return SP's output result. But I'm having problems to mock SP's output for unit testing. P.S. I'm using MOQ framework for mocking.