plt.savefig produces blank figure in Python

I want to save a plot as a png. The following code produces a blank figure:

import numpy as np
import matplotlib.pyplot as plt
x = np.linspace (0 , 10, 1000)
y = x**2
plt.plot(x, y)
plt.savefig('line_plot.png', dpi=100)

I am using Python 3.6 together with Anaconda and Spyder 3.2.0. How can I get a png containing the desired plot?