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Description
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I have checked that this issue has not already been reported.
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I have confirmed this bug exists on the latest version of pandas.
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I have confirmed this bug exists on the master branch of pandas.
Reproducible Example
import pandas as pd
df = pd.DataFrame({"date": [pd.NaT], "value": [11]})
grouper = pd.Grouper(freq="M", key="date")
>>> gb = df.groupby(grouper)
[...]
File "pandas/core/resample.py", line 1878, in _get_timestamp_range_edges
first = first.normalize()
AttributeError: 'NaTType' object has no attribute 'normalize'
### Issue Description
Example derived from test_timegrouper_apply_return_type_series
### Expected Behavior
Either not raise or raise at construction of the Grouper object with a useful exception message.
### Installed Versions
master