python - vectorize returns 0s instead of results - Stack Overflow

The MWE below shows that vectorize returns zeros instead of the actual values. Compare the printout from within the function to the arrays returned in the second test. It is curious that the first test with hollow=False returns the correct result. Is this a bug or a not so obvious limitation of vectorize?

In [1]: run potentials-test.py                                                                                                           
potential r in < R: 0.2   -0.0  False
potential r in < R: 0.2   -0.0  False
potential r in < R: 0.4   -0.1  False
potential r in < R: 0.6   -0.2  False
potential r in < R: 0.8   -0.3  False
potential r in >= R: 1.0   -1.0
potential r in >= R: 1.2   -0.8
potential r in >= R: 1.4   -0.7
potential r in >= R: 1.6   -0.6
potential r in >= R: 1.8   -0.6
potential r in >= R: 2.0   -0.5
[-0.02       -0.08       -0.18       -0.32       -1.         -0.83333333
 -0.71428571 -0.625      -0.55555556 -0.5       ]

potential r in < R: 0.2   -1.0  True
potential r in < R: 0.2   -1.0  True
potential r in < R: 0.4   -1.0  True
potential r in < R: 0.6   -1.0  True
potential r in < R: 0.8   -1.0  True
potential r in >= R: 1.0   -1.0
potential r in >= R: 1.2   -0.8
potential r in >= R: 1.4   -0.7
potential r in >= R: 1.6   -0.6
potential r in >= R: 1.8   -0.6
potential r in >= R: 2.0   -0.5
[-1 -1 -1 -1 -1  0  0  0  0  0]

The MWE code is:

from numpy import *
from math import pi

class sphere():
  def __init__(self, GM=1, R=1, hollow=True):
    self.GM = GM 
    self.R = R
    self.Grho = (3/(4*pi))*GM/R**3
    self.hollow = hollow
    self.vpotential = vectorize(self.potential)
    
  def potential(self, r):
     if r >= self.R:
       res = -self.GM*self.R/r
       print("potential r in >= R: %3.1f  %5.1f"% (r, res))
       return res
     else:
       if self.hollow:
         res = -self.GM
         print("potential r in < R: %3.1f  %5.1f "% (r, res), self.hollow)
         return res
       else:
         res = -(2*pi/3)*self.Grho*r**2/self.R**2
         print("potential r in < R: %3.1f  %5.1f "% (r, res), self.hollow)
         return res

x=linspace(0.2,2,10) 

s = sphere(hollow=False)
print(s.vpotential(x))
h = sphere(hollow=True)
print(h.vpotential(x))