how to deal with SExpressionEvaluationException

A SExpressionEvaluationException is thrown when i set in General->Message content :

bayer-->setItemValue-->itemValue

Declared return type class pro.bonita.Bayer is not compatible with evaluated type class java.lang.Double for expression itemValue

my class:

public class Bayer implements serializable
{
private static final long serialVersionUID = 1L;
Double itemValue = 0;
private void setItemValue(Double itemValue)
{this.itemValue = itemValue;
}
private Double getItemValue()
{
return itemValue;
}
any Help?