BUG #13619: regression functions return Null

The following bug has been logged on the website:

Bug reference: 13619
Logged by: Biswadeep Banerjee
Email address: biswadeep(dot)banerjee(at)actifio(dot)com
PostgreSQL version: 9.4.4
Operating system: Any
Description:

Hi,
Trending analysis is one of our use case, where I am trying the use the
regression function in postgres 9.4.4.
eg. regr_slope(y,x); regr_intercept(y,x).

I have tried to use the functions to fit in our trending analysis SQL but
returns NULL.
As test also tried with constant values, like,
select regr_slope(4,3), regr_intercept(4,3) from table;

--- returns NULL.

Is this a known issue? or I am using the function differently.

Thanks