Question: What is so special about the function in a fieldplot?

Simple issue. I think it is a bug, but if not, it tends to cause logic problems with new users of Maple.

with(plots):
z := A*x*y:
A:=1.0:
plot3d(z, x=0..1, y=0..1); # works as expected
contourplot(z, x=0..1, y=0..1); # works
fieldplot(z, x=0..1, y=0..1); 
    --> Error, (in plots/fieldplo) invaled 1st argument (the function) 1.0*x*y

Is there something special about what the fieldplot expects for the function? I don't see it in the help. Or is this a bug?  

Yes, there is a work around, define all constants first

A:=1.0:   z := A*x*y: fieldplot(z, x=0..1, y=0..1); 

But I fail to understand why my example should fail. (Note, I have seen this situation for every version, so it is not new.)

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