Xlib: extension "XInputExtension" missing on display "ubuntu.local:1.0".

I am running matplotlib applications over VNC and during the
initialization segment, I always see the above error. It doesn't seem
to cause any problems, but I am curious what is causing it. Does
anyone have an idea? This happens using the Qt4Agg backend at least.
It seems to occur right when I first call ax.plot(...)