command:<mkdocs serve>error TypeError: unsupported operand type(s) for +: 'int' and 'str'

Question

when I use this command pip install mkdocs mkdocs-materialand mkdocs serveto run a simple project ,it throws an errorTypeError: unsupported operand type(s) for +: 'int' and 'str':

python version is 3.8.5, pip version ispip 21.1 from /home/linxianmiao/.local/lib/python3.8/site-packages/pip (python 3.8)