Python groupbyメソッド実行時エラー

ValueError                                Traceback (most recent call last)
<ipython-input-6-727f137a9620> in <module>
----> 1 customer_clustering.groupby("cluster").count()

~\Anaconda3\lib\site-packages\pandas\core\frame.py in groupby(self, by, axis, level, as_index, sort, group_keys, squeeze, observed, dropna)
   6509         axis = self._get_axis_number(axis)
   6510 
-> 6511         return DataFrameGroupBy(
   6512             obj=self,
   6513             keys=by,

~\Anaconda3\lib\site-packages\pandas\core\groupby\groupby.py in __init__(self, obj, keys, axis, level, grouper, exclusions, selection, as_index, sort, group_keys, squeeze, observed, mutated, dropna)
    523             from pandas.core.groupby.grouper import get_grouper
    524 
--> 525             grouper, exclusions, obj = get_grouper(
    526                 obj,
    527                 keys,

~\Anaconda3\lib\site-packages\pandas\core\groupby\grouper.py in get_grouper(obj, key, axis, level, sort, observed, mutated, validate, dropna)
    796         # allow us to passing the actual Grouping as the gpr
    797         ping = (
--> 798             Grouping(
    799                 group_axis,
    800                 gpr,

~\Anaconda3\lib\site-packages\pandas\core\groupby\grouper.py in __init__(self, index, grouper, obj, name, level, sort, observed, in_axis, dropna)
    515                 if getattr(self.grouper, "ndim", 1) != 1:
    516                     t = self.name or str(type(self.grouper))
--> 517                     raise ValueError(f"Grouper for '{t}' not 1-dimensional")
    518                 self.grouper = self.index.map(self.grouper)
    519                 if not (

ValueError: Grouper for 'cluster' not 1-dimensional