python pandas学習中の初心者です。
https://note.nkmk.me/python-pandas-where-mask/

こちらのサイトを参考にして print(df < 0) を実行したところ、下記のエラーが表示されます。
回避方法をご教授お願いします。

DataFremeデータサンプル

>>> df
    A  B  C   D
0 -20  1  a -20
1 -10  2  b   2
2   0  3  b   3
3  10  4  a  10
4  20  5  c   5

エラーが出る部分

>>> print(df < 0)
Traceback (most recent call last):
  File "<pyshell#71>", line 1, in <module>
    print(df < 0)
  File "C:\Users\XXXXX\AppData\Local\Programs\Python\Python37-32\lib\site-packages\pandas\core\ops\__init__.py", line 1579, in f
    res = self._combine_const(other, func)
  File "C:\Users\XXXXX\AppData\Local\Programs\Python\Python37-32\lib\site-packages\pandas\core\frame.py", line 5418, in _combine_const
    return ops.dispatch_to_series(self, other, func)
  File "C:\Users\XXXXX\AppData\Local\Programs\Python\Python37-32\lib\site-packages\pandas\core\ops\__init__.py", line 596, in dispatch_to_series
    new_data = expressions.evaluate(column_op, str_rep, left, right)
  File "C:\Users\XXXXX\AppData\Local\Programs\Python\Python37-32\lib\site-packages\pandas\core\computation\expressions.py", line 221, in evaluate
    return _evaluate(op, op_str, a, b, **eval_kwargs)
  File "C:\Users\XXXXX\AppData\Local\Programs\Python\Python37-32\lib\site-packages\pandas\core\computation\expressions.py", line 70, in _evaluate_standard
    return op(a, b)
  File "C:\Users\XXXXX\AppData\Local\Programs\Python\Python37-32\lib\site-packages\pandas\core\ops\__init__.py", line 570, in column_op
    return {i: func(a.iloc[:, i], b) for i in range(len(a.columns))}
  File "C:\Users\XXXXX\AppData\Local\Programs\Python\Python37-32\lib\site-packages\pandas\core\ops\__init__.py", line 570, in <dictcomp>
    return {i: func(a.iloc[:, i], b) for i in range(len(a.columns))}
  File "C:\Users\XXXXX\AppData\Local\Programs\Python\Python37-32\lib\site-packages\pandas\core\ops\__init__.py", line 1229, in wrapper
    res = na_op(values, other)
  File "C:\Users\XXXXX\AppData\Local\Programs\Python\Python37-32\lib\site-packages\pandas\core\ops\__init__.py", line 1091, in na_op
    result = _comp_method_OBJECT_ARRAY(op, x, y)
  File "C:\Users\XXXXX\AppData\Local\Programs\Python\Python37-32\lib\site-packages\pandas\core\ops\__init__.py", line 1069, in _comp_method_OBJECT_ARRAY
    result = libops.scalar_compare(x, y, op)
  File "pandas/_libs/ops.pyx", line 96, in pandas._libs.ops.scalar_compare
TypeError: '<' not supported between instances of 'str' and 'int'