Question: Implicitplot empty, Why?

Why does the implicit plot return empty?

plots:-implicitplot((x^2+y^2 = 1)^2, x = -3 .. 3, y = -3 .. 3);# plots
   plots:-implicitplot((x^2+y^2-1)^2, x = -3 .. 3, y = -3 .. 3) # empty plot

 

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