Why am I getting this error - KeyError: ('datetime64[ns]', 'left')? - python-3.x

Question

When I run the code below this error is displayed:
KeyError: ('datetime64[ns]', 'left')
Anyone have a answer ? Please find the code below:
def transform_in_intervals(df,freq=to_offset('5t')):
df = df.copy()
time = df.index
interval = pd.interval_range(start=time[0],periods=(time[-1]-time[0])/pd.Timedelta(freq)+1,freq=freq,closed="left")
df.reset_index(False,inplace=True)
i, c = 0, 0
while i < len(df):
if df.loc[i,"time"] in interval[c]:
df.loc[i,"interval"] = interval[c]
i += 1
else:
c += 1
df_res = pd.DataFrame([])
for iv, df_left in df.groupby("interval"):
df_res = df_res.append(df_left.drop("time",axis=1).mean(),ignore_index=True)
df_res.set_index(interval,inplace=True)
return df_res
start= time.time()
trans_df = transform_in_intervals(merged_df)
end = time.time()
print("time:{:.3f} s".format(end-start))
KeyError Traceback (most recent call last)
<ipython-input-31-bfcffbdad696> in <module>()
1 start= time.time()
----> 2 trans_df = transform_in_intervals(merged_df)
3 end = time.time()
4 print("time:{:.3f} s".format(end-start))
<ipython-input-30-2d97dbfdb4d1> in transform_in_intervals(df, freq)
14 for iv, df_left in df.groupby("interval"):
15 df_res = df_res.append(df_left.drop("time",axis=1).mean(),ignore_index=True)
---> 16 df_res.set_index(interval,inplace=True)
17 return df_res
~\AppData\Local\Continuum\anaconda2\envs\py35\lib\site-packages\pandas\core\frame.py in set_index(self, keys, drop, append, inplace, verify_integrity)
3921
3922 # clear up memory usage
-> 3923 index._cleanup()
3924
3925 frame.index = index
~\AppData\Local\Continuum\anaconda2\envs\py35\lib\site-packages\pandas\core\indexes\base.py in _cleanup(self)
1912
1913 def _cleanup(self):
-> 1914 self._engine.clear_mapping()
1915
1916 #cache_readonly
pandas\_libs\properties.pyx in pandas._libs.properties.CachedProperty.__get__()
~\AppData\Local\Continuum\anaconda2\envs\py35\lib\site-packages\pandas\core\indexes\interval.py in _engine(self)
366 #cache_readonly
367 def _engine(self):
--> 368 return IntervalTree(self.left, self.right, closed=self.closed)
369
370 #property
pandas\_libs\intervaltree.pxi in pandas._libs.interval.IntervalTree.__init__()
KeyError: ('datetime64[ns]', 'left')