The source code for argmin() and argmax() purports to handle NULL objects (for object arrays), but the code handling them is buggy:

a = np.empty(4, dtype='O')
ctypes.memset(a.ctypes.data, 0, a.nbytes)
a[1] = 10
a[3] = 30

print(np.argmin(a))  # should be 1, returns 0
print(np.argmax(a))  # should be 3, returns 0

min() and max() take another approach: they interpret the NULL pointers as references to None.