Why does the following code produce an error when plotting a 3D bar chart?

Why does the following code produce an error when plotting a 3D bar chart?

import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
x = [1, 2, 3]
y = [1, 2]
z = [0, 0, 0]
dx = dy = 1
dz = [4, 5, 6]
ax.bar3d(x, y, z, dx, dy, dz)
plt.show()