can only concatenate str (not "numpy.float64") to str

Ошибка: can only concatenate str (not "numpy.float64") to str

from pandas import read_csv
from pandas import datetime
from matplotlib import pyplot

def parser(x):
    return datetime.strptime('190'+x, '%Y-%m')

series = read_csv('shampoo_sales.csv', header=0, parse_dates=[0], index_col=0, squeeze=True, date_parser=parser)
print(series.head())
series.plot()
pyplot.show()