TabularPandas error: nan values in `MachineHoursCurrentMeter`


This is regarding the Tabular Modeling chapter and the Blue Book for Bulldozers dataset. I’ve constantly been getting an error when I set up my TabularPandas as per the book.

After following every step in the book, I type in:
to = TabularPandas(df, procs, cat, cont, y_names=dep_var, splits=splits)

And the error I get is the following:
nan values in MachineHoursCurrentMeter but not in setup training set

Can someone please help me understand what this error means and how do I solve it?