def count_nulls(self, field):
        """
        Count the number of null values in a column
        """
        try:
            n = self.df[field].isnull().sum()
        except KeyError:
            self.warning("Can not find column", field)
            return
        except Exception as e:
            self.err(e, "Can not count nulls")
            return
        self.ok("Found", n, "nulls in column", field)