Python3-ValueError:not enough values to unpack (expected 2, got 0)

Python3-valueError :not enough values to unpack (expected 2, got 0)
Specific errors are as follows:

Traceback (most recent call last):
  File "/Users/zhangsf/code/python/my-project/", line 83, in <module>
    xy1.boxplot(column='ranking', by='subsidy')
  File "/Users/zhangsf/anaconda3/lib/python3.7/site-packages/pandas/plotting/", line 2254, in boxplot_frame
    return_type=return_type, **kwds)
  File "/Users/zhangsf/anaconda3/lib/python3.7/site-packages/pandas/plotting/", line 2223, in boxplot
  File "/Users/zhangsf/anaconda3/lib/python3.7/site-packages/pandas/plotting/", line 2682, in _grouped_plot_by_column
    keys, values = zip(*gp_col)
ValueError: not enough values to unpack (expected 2, got 0)

It means that python expected there to be two return values from zip(), but there were none.
Error: This means that Python expects zip () to return two values, but neither
The error statement is:

xy1.boxplot(column='ranking', by='subsidy')

The reason is that there is a problem with the imported data, which causes the ranking of the column to be NaN and no return value
Just re-introduce the correct data

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