[Bug]: Unable to pickle figure with aligned labels

import matplotlib.pyplot as plt
import pickle

fig = plt.figure()
ax1 = fig.add_subplot(211)
ax2 = fig.add_subplot(212)
time=[0,1,2,3,4]
speed=[40000,4300,4500,4700,4800]
acc=[10,11,12,13,14]
ax1.plot(time,speed)
ax1.set_ylabel('speed')
ax2.plot(time,acc)
ax2.set_ylabel('acc')

fig.align_labels() ##pickling works after removing this line 

pickle.dumps(fig)
plt.show()

align.py", line 16
pickle.dumps(fig)
TypeError: cannot pickle 'weakref.ReferenceType' object