On 2.0.x: ``` df = pd.DataFrame({'a': [1, 2, 3], 'b': [3, 4, 5], 'c': 6, 'd': 7}).set_index(['a', 'b', 'c']) ser = pd.Series(8, index=df.index.droplevel('c')) print(df) # d # a b c # 1 3 6 7 # 2 4 6 7 # 3 5 6 7 print(ser) # a b # 1 3 8 # 2 4 8 # 3 5 8 # dtype: int64 df['d'] = ser print(df) # d # a b c # 1 3 6 8 # 2 4 6 8 # 3 5 6 8 ``` On main and 2.1.0rc this raises: > AssertionError: Length of new_levels (2) must be same as self.nlevels (3) A git bisect points at ``` commit ac3153b30a611d2593b002ca5d9ff22a2bb30af0 Author: Luke Manley <lukemanley@gmail.com> Date: Mon Jul 10 13:51:37 2023 -0400 PERF: MultiIndex set and indexing operations (#53955) * PERF: MultiIndex set and indexing ops * whatsnew * convert to generator doc/source/whatsnew/v2.1.0.rst | 1 + pandas/_libs/index.pyx | 17 +++++++++-------- pandas/core/indexes/multi.py | 13 +++++++++++++ 3 files changed, 23 insertions(+), 8 deletions(-) ``` cc @lukemanley