Bug report

Bug summary

Matplotlib shows a warning when adding a legend to a graph while using the TeX backend.
Error is: findfont: Font family ['serif'] not found. Falling back to DejaVu Sans.
A typical looking graph is supplied but I'm unsure as to the correctness of the legend font.

Code for reproduction

from matplotlib import rc
import matplotlib.pyplot as plt

rc('font', **{'family':'serif','serif':'Computer Modern'})
rc('text', usetex=True)

plt.figure()
plt.plot(0,0,label='Input')
plt.title('Commenting out legend removes warning')

plt.legend()

plt.show()

Actual outcome

findfont: Font family ['serif'] not found. Falling back to DejaVu Sans.

The graph is output normally.

Expected outcome

No errors, and a key that works.

Matplotlib version

• Operating system: Ubuntu Focal Fossa GNOME Terminal 3.36.1.1
• Matplotlib version: 3.2.1
• Matplotlib backend (print(matplotlib.get_backend())): GTK3Agg
• Python version: 3.8.2 (GCC 9.3.0 on linux)
• Jupyter version (if applicable): NA
• Other libraries:

matplotlib was installed via pip (20.1.1), and python was installed via apt.