title: encounter error 'str' object has no attribute 'log' when perform np.log()
body: <p>after I set date to date object and make it index, when I use np.log(), i got the error message:
  AttributeError:'str' object has no attribute 'log'
  TypeError: loop of ufunc does not support argument 0 of type str which has no callable log method
  my input value is a series of data, which shouldn't be a problem
  <a href="https://i.stack.imgur.com/GjhWA.jpg" rel="nofollow noreferrer">python codes</a></p>