BUG: failed to aggregate

[wxinl]

• 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 master branch of pandas.

Reproducible Example

import pandas as pd
df1 = pd.DataFrame(dict2 = {'index': {0: 7158892,1: 7158893,2: 7158894,3: 7158895,4: 7158896,5: 7158897,6: 7158898},
 'uid': {0: 1081156,1: 1081156,2: 1081156,3: 1081156,4: 1081156,5: 1081156,6: 1081156},
 'lat': {0: 37.524228,1: 37.546708,2: 37.433964,3: 38.245017,4: 37.546708,5: 37.500817,6: 37.433964},
'lng': {0: -121.94779,1: -122.069067,2: -121.886765,3: -121.455548,4: -122.069067,5: -121.981362,6: -121.886765}})

agg1 = df1.groupby(['uid']).agg(pd.Series.mode)

Issue Description

ValueError: could not broadcast input array from shape (7,4) into shape (7,)

Expected Behavior

expect something like this:
1081156,[37.433964 37.546708],[-121.886765 -122.069067]

Installed Versions

INSTALLED VERSIONS ------------------ commit : 945c9ed python : 3.9.7.final.0 python-bits : 64 OS : Linux OS-release : 5.4.0-81-generic Version : #91~18.04.1-Ubuntu SMP Fri Jul 23 13:36:29 UTC 2021 machine : x86_64 processor : x86_64 byteorder : little LC_ALL : None LANG : en_US.UTF-8 LOCALE : en_US.UTF-8

pandas : 1.3.4
numpy : 1.21.2
pytz : 2021.1
dateutil : 2.8.2
pip : 21.2.4
setuptools : 58.0.4
Cython : None
pytest : None
hypothesis : None
show more (open the raw output data in a text editor) ...

tabulate : None
xarray : 0.19.0
xlrd : None
xlwt : None
numba : None

[mzeitlin11]

Thanks for reporting this @wxinl! Probably the same as #38534?

[mzeitlin11]

Closing in favor of the other issue

[wxinl]

Solved by adding a pseudo row at the top of the dataframe to initialize the aggregation