I am trying to use the

You can see from the source that the YearPicker doesn't have this problem because it passes every props down manually, while the MonthPicker chooses to spread the remaining props which includes renderInput - this is an invalid prop because the HTML attribute doesn't know anything about JS callback object.

This error is just a warning from ReactJS when it thinks you're doing something wrong, but you're not because this is an upstream bug, and it doesn't affect anything else functionality-wise, so ignore it.