How to add together two columns in pandas without receiving SettingWithCopyWarning

Trying to add add one column to another using pandas with:

merged_df.loc[:, 'airline_name'] = merged_df.loc[:, 'airline_name'] + merged_df['airline_icao_unique_code']

but every time I try I receive the warning :

SettingWithCopyWarning:
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead
See the caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
self.obj[selected_item_labels] = value
Process finished with exit code 0

Is there any way to do this without triggering this warning? I have tried altering the syntax and creating temporary variables but I cant seem to get it to work. I have also tried using the .copy() function but still no dice.