diff --git a/modin/backends/pandas/query_compiler.py b/modin/backends/pandas/query_compiler.py
index 26d85231645..aa07a223120 100644
--- a/modin/backends/pandas/query_compiler.py
+++ b/modin/backends/pandas/query_compiler.py
@@ -2565,20 +2565,17 @@ def groupby_agg_builder(df, by=None, drop=False, partition_idx=None):
             missmatched_cols = pandas.Index([])
             if by is not None:
                 internal_by_df = by[internal_by]
+
                 if isinstance(internal_by_df, pandas.Series):
                     internal_by_df = internal_by_df.to_frame()
 
-                if isinstance(internal_by_df, pandas.DataFrame):
-                    missmatched_cols = internal_by_df.columns.difference(df.columns)
-                    df = pandas.concat(
-                        [df, internal_by_df[missmatched_cols]],
-                        axis=1,
-                    )
-                    internal_by_cols = internal_by_df.columns
-                elif is_multi_by:
-                    internal_by_cols = pandas.Index([internal_by_df.name])
-                else:
-                    internal_by_cols = pandas.Index([internal_by_df])
+                missmatched_cols = internal_by_df.columns.difference(df.columns)
+                df = pandas.concat(
+                    [df, internal_by_df[missmatched_cols]],
+                    axis=1,
+                    copy=False,
+                )
+                internal_by_cols = internal_by_df.columns
 
                 external_by = by.columns.difference(internal_by)
                 external_by_df = by[external_by].squeeze(axis=1)