不能在python virtualenv中使用pip

我创建了一个名为 proj_env 并在Windows命令提示下激活,如下所示:

C:\Users\me\Desktop\loan_predictor\proj_env\Scripts>activate.bat

(proj_env) C:\Users\me\Desktop\loan_predictor\proj_env\Scripts>pip3 install flask

Traceback (most recent call last):
  File "D:\Python\Python35\Lib\runpy.py", line 184, in _run_module_as_main
    "__main__", mod_spec)
  File "D:\Python\Python35\Lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Users\me\Desktop\loan_predictor\proj_env\Scripts\pip3.exe\__main__.py", line 5, in <module>
ImportError: No module named 'pip._internal'

我不知道为什么 pip 和 pip3 不起作用,虽然我起作用的时候 pip3 install virtualenv .

我错过了什么?