Does anyone know why the following returns a run-time error?

Range("csv1").FormulaR1C1 =
"=IF(ISERROR(DATEVALUE(RC[19])),RC[19],DATEVALUE([RC[19]))"

I have tried ISERR Function, but still no joy. This formula definitely
works when entered manually.

Would be grateful for any words of wisdom. Thanks.

Toyin.