Why does this code raise an error?

Why does this code raise an error?

import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D

fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
X, Y = np.meshgrid(np.linspace(-1, 1, 10), np.linspace(-1, 1, 10))
Z = np.sin(X) + np.cos(Y)
ax.plot_wireframe(X, Y)
plt.show()