Select query errors when changed to Update

[Middlemarch]

SELECT tblMain.Prefix, Len([Comments]) AS Expr1, tblMain.SheetSort
FROM tblMain


WHERE (((Len([Comments]))>0 And (Len([Comments]))<5) AND ((tblMain.Sheet)=2));

If I run this as an Update query it tells me Len([Comments]) is not a valid name.
Is there some other way to do this ? I want to update to "" or vbNull.

Field Comments is a Long Text datatype and does have text but some numbers have got in which I want to remove.

[June7]

I can UPDATE long text field.

Show your UPDATE SQL.