>Creating an index using the QUARTER function on an empty table in VFP 7 generates a "Function argument value, type or count is invalid" error message.
>
>For example:
>CREATE TABLE x (xdate D)
>INDEX ON QUARTER(xdate) TAG qtr

The QUARTER() function generates an error if it's argument is an empty date or Null. Try to use

INDEX ON IIF(Empty(xdate) Or IsNull(xdate), 0, QUARTER(xdate)) TAG qtr

In order to determin the size of the index VFP has to evaluate index expression. VFP uses either the first physical record in the table, if there're any records, or empty record to do that. It's similar to the way VFP determines the size of the fields in the Select Sql.