BUG: Did not expect new dtype datetime64[ns] to equal self.dtype datetime64[ns]

[StefanFrey]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

change_date["latest_change_at"].fillna(np.datetime64('2001-01-01T00:00:00'), inplace=True)


AssertionError: Did not expect new dtype datetime64[ns] to equal self.dtype datetime64[ns]. Please report a bug at https://github.com/pandas-dev/pandas/issues.

Issue Description

Original code was:
change_date["latest_change_at"].fillna("2001-01-01T00:00:00Z", inplace=True)
This is my error:
Value '2001-01-01T00:00:00Z' has dtype incompatible with datetime64[ns], please explicitly cast to a compatible dtype first.

So I tried to change the values for the fillna to datetime64[ns] by running this:
change_date["latest_change_at"].fillna(np.datetime64('2001-01-01T00:00:00'), inplace=True)

Expected Behavior

This should cast.

Installed Versions

Pandas 2.1.4

[phofl]

Hi, thanks for your report. Can you give us a full reproducer that includes the construction of change_date?

[mroeschke]

Closing as this issue needs more information to reproduce

[thecurve8]

import pandas as pd

df = pd.DataFrame({
'datetime' : pd.date_range('1/1/2011', period=3, freq='H'),
'value' : [1,2,3]
})

df.iloc[0,0] = None
df.fillna(pd.Timestamp('0001-01-01'), inplace=True)