title: Pandas to_datetime assertion error is throwing an error
body: <p>My python is throwing an assertion error when converting a date in string format to a datetime format. This is being used in 'read_csv" as a converter.</p>
  
  <p>For example my data looks like this: <code>"01-SEP-18 01.30.30.000000 AM"</code></p>
  
  <p>As far as I can tell the format should be the below.  This is not my exact code, but I included the string rather than expressing my converter.  I am aware that to_datetime is relatively smart and tried without a format only to receive a similar/same error.</p>
  
  <pre><code>pn.to_datetime('01-SEP-18 01.30.30.000000 AM','%d-%b-%y %I.%M.%S.%f %p')
  pn.to_datetime('01-SEP-18 01.30.30.000000 AM')
  
  
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "C:\ProgramData\Anaconda3\lib\site-packages\pandas\core\tools\datetimes.py", line 469, in to_datetime
      result = _convert_listlike(np.array([arg]), box, format)[0]
    File "C:\ProgramData\Anaconda3\lib\site-packages\pandas\core\tools\datetimes.py", line 368, in _convert_listlike
      require_iso8601=require_iso8601
    File "pandas\_libs\tslib.pyx", line 492, in pandas._libs.tslib.array_to_datetime
    File "pandas\_libs\tslib.pyx", line 513, in pandas._libs.tslib.array_to_datetime
  
  AssertionError
  </code></pre>