ireport - Error when using Print When Expression

I'm having a problem while generating the report, the print when expression function does not work, the report always shows this line with the value null

I'm doing this on the line

<printWhenExpression><![CDATA[new Boolean($F{codProduto} != null)]]></printWhenExpression>

But always the report appears in this line in cinza written null :

How can I resolve this situation?