Replacing "x" is not right!

[Spybot]

Hello roadrunner!
That's what I'm talking about, the Prime CAS takes "x" as a zero, but I haven't assigned any numeric value to it. if I use any other variable name... like "r" or "y" it does the replacement correctly, the problem comes in when replacing the "same" variable name.
And... yes! subst() certainly gets the job done, but I think that particular "constrain operator" should do it too.