What is the coefficient of correlation(r) when slope is 0 and there are 2 points?

[ROAD_DOGG33J]

Let's say we have two data points (4,1) and (8,1). What would r be for that data?

thanks

edit: Actually would the answer be that r can't be applied to this situation?

 

[ROAD_DOGG33J]

The answer for multiple points that are horizontal is r=0, at least for the purposes of the test I took. So I guess the same could be said for 2 points.