Odd group by clause error

Hi EE,

The SQL Query i have written:

SELECT
'DB_NAME' = db.name,
'FILE_NAME' = mf.name,
'FILE_TYPE' = mf.type_desc,
'FILE_PATH' = mf.physical_name,
'Size in MB' = CAST(((SUM(mf.size)* 8) / 1024.0) AS DECIMAL(18,2) )
FROM
sys.databases db
INNER JOIN sys.master_files mf
ON db.database_id = mf.database_id
Where DB.state = 0 
Group by  GROUPING SETS
              (
                     (db.name, Type_Desc),
                     (mf.size)
              )

Select allOpen in new window

Error: Column 'sys.master_files.name' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

I don't understand how this error is occurring can someone assist any help is welcome.

Thank you.