Estoy luchando con lo que parece ser un problema muy fácil: cómo hacer que Seaborn trace un gráfico de líneas de series de tiempo desde un marco de datos de pandas. ¿Qué estoy haciendo mal aquí?

import seaborn as sns
import pandas as pd
df=pd.DataFrame({"Date":["2015-03-03","2015-03-02","2015-03-01"],"Close":[1,3,2]})
df["Date"]=pd.to_datetime(df["Date"])#Not sure if seaborn can parse strings as dates
sns.tsplot(data=df,unit=None, time="Date", value="Close")

Me sale este rastreo:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-306-20e252f661c2> in <module>()
      1 df=pd.DataFrame({"Date":["2015-03-03","2015-03-02","2015-03-01"],"Close":[1,3,2]})
      2 df["Date"]=pd.to_datetime(df["Date"])
----> 3 sns.tsplot(data=df,unit=None, time="Date", value="Close")

C:\Anaconda\lib\site-packages\seaborn\timeseries.pyc in tsplot(data, time, unit, condition, value, err_style, ci, interpolate, color, estimator, n_boot, err_palette, err_kws, legend, ax, **kwargs)
    275     for c, (cond, df_c) in enumerate(data.groupby(condition, sort=False)):
    276 
--> 277         df_c = df_c.pivot(unit, time, value)
    278         x = df_c.columns.values.astype(np.float)
    279 

C:\Anaconda\lib\site-packages\pandas\core\frame.pyc in pivot(self, index, columns, values)
   3507         """
   3508         from pandas.core.reshape import pivot
-> 3509         return pivot(self, index=index, columns=columns, values=values)
   3510 
   3511     def stack(self, level=-1, dropna=True):

C:\Anaconda\lib\site-packages\pandas\core\reshape.pyc in pivot(self, index, columns, values)
    324     else:
    325         indexed = Series(self[values].values,
--> 326                          index=MultiIndex.from_arrays([self[index],
    327                                                        self[columns]]))
    328         return indexed.unstack(columns)

C:\Anaconda\lib\site-packages\pandas\core\frame.pyc in __getitem__(self, key)
   1795             return self._getitem_multilevel(key)
   1796         else:
-> 1797             return self._getitem_column(key)
   1798 
   1799     def _getitem_column(self, key):

C:\Anaconda\lib\site-packages\pandas\core\frame.pyc in _getitem_column(self, key)
   1802         # get column
   1803         if self.columns.is_unique:
-> 1804             return self._get_item_cache(key)
   1805 
   1806         # duplicate columns & possible reduce dimensionaility

C:\Anaconda\lib\site-packages\pandas\core\generic.pyc in _get_item_cache(self, item)
   1082         res = cache.get(item)
   1083         if res is None:
-> 1084             values = self._data.get(item)
   1085             res = self._box_item_values(item, values)
   1086             cache[item] = res

C:\Anaconda\lib\site-packages\pandas\core\internals.pyc in get(self, item, fastpath)
   2858                         loc = indexer.item()
   2859                     else:
-> 2860                         raise ValueError("cannot label index with a null key")
   2861 
   2862             return self.iget(loc, fastpath=fastpath)

ValueError: cannot label index with a null key

Desafortunadamente, no he encontrado ninguna respuesta en la documentación.