Inge,
VFP's SQL parser evaluates calls to UDFs before running the actual query. It does this in order to know the data type and size of the return value. Could it be that your parameters are not initialized correctly? What are parm1 and parm2?

Could you post the code of the 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!