Update

Author: yuanx749

pandas/core/frame.py

@@ -8671,9 +8671,7 @@ def combine(
         2  NaN  3.0 1.0
         """
         other_idxlen = len(other.index)  # save for compare
-
-        # preserve column order
-        new_columns = self.columns.union(other.columns, sort=False)
+        other_columns = other.columns
 
         this, other = self.align(other)
         new_index = this.index
@@ -8684,6 +8682,8 @@ def combine(
         if self.empty and len(other) == other_idxlen:
             return other.copy()
 
+        # preserve column order
+        new_columns = self.columns.union(other_columns, sort=False)
         do_fill = fill_value is not None
         result = {}
         for col in new_columns: