Unexpected singular point

Dear all,
I am a newcomer to Axiom, please pardon me if my question is trivial. Here
follows the dialogue with Axiom I do not understand:

(1) -> p:=((x**2+y**2)**2-x*y)

         4     2 2          4
   (1)  y  + 2x y  - x y + x
                                                     Type: Polynomial
Integer

(2) -> draw(p=0,x,y,range==[0..0.5,0..0.5])

   >> Error detected within library code:
   singular pts in region of sketch

I cannot see the singular point in "p". Any help?

Francesco

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