Question: Strange error message using plot3d

When trying to perform the following:

p1 := proc (x, y) if x^2+y^2 <= 1 then x*y-y^2 else 0 end if end proc;
plot3d(p1, x = -1 .. 1, y = -1 .. 1);
Error, (in plot3d) expected ranges but received x = -1 .. 1 and y = -1 .. 1
 

I get this strange error message. To the best of my knowledge x and y ARE provided as ranges. What am I missing/not understanding?

If I omit the ranges in plot3d Maple returns a correct plot, but the default range (-10 .. 10) does not display sufficient details

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