A complex sql issue

I am trying to update a file and could use some help constructing the SQL
statement.

Two files:

File 1 - Work file
Fld1
Fld2
Fld3
Fld4
Fld5
Fld6
Fld7
Fld8
Fld9
Fld10

File 2 - Production File
Fld1
Fld2
Fld3
Fld4
Fld5
Fld6
Fld7
Fld8
Fld9
Fld10

Both files have the exact same layout, I need to update fields 5,6,7 in
the production file with the data in the work file where flds1-10,
excluding 5-7, are an exact match.

When I first tried it I got a -206 error, so I know that I had the update
statement written incorrectly. Here is what I was trying to use:

Update File2 as a
Set a.Fld5 = b.Fld5,
a.Fld6 = b.Fld6,
a.Fld7 = b.Fld7
Where exists
(select * from File1 as b
Where
a.Fld1 = b.Fld1 and
a.Fld2 = b.Fld2 and
a.Fld3 = b.Fld3 and
a.Fld4 = b.Fld4 and
a.Fld8 = b.Fld8 and
a.Fld9 = b.Fld9 and
a.Fld10 = b.Fld10);

I'm sure the reason its blowing the -206 is because on the update side of
the statement b doesn't yet exist.

Any help would be appreciated.