As explained in mskb #Q133001, VFP uses the first record of the table to create and size output columns regardless of SELECT WHERE clause. You've to account for that extra call when you create an UDF.

>I have a UDF called myUDF, which returns a numeric data type.
>
>?myUDF(parm1,parm2)
>correctly returns a numeric data.
>
>SELECT myUDF(parm1,parm2) FROM myTable
>gives me a 'Data Type Mismatch' error.
>
>I've stepped through the UDF & all is fine until I hit the last statement:
>RETURN xRet
>(where xRet is the return value).
>
>Anybody knows what's wrong?
>Thanks all!