javax.xml.xpath.XPathExpressionException: org.apache.xpath.domapi.XPathStylesheetDOM3Exception: Prefix must resolve to a namespace

I am using the following condition on Apache Camel Java DSL:

.choice().when().xpath("//*:RequestField1 > //*:RequestField2").setProperty("Result").xpath("//*:RequestField1").endChoice()

The error above appears on runtime.