You get this message when Struts is unable to map the data in the HTML form to the properties in your ActionForm bean. Make sure each of the properties on your bean is either a String or a boolean. Do you have any properties of type java.util.Date or other objects? That might cause this error. Also check to see that you have public getters and setter for each of your properties.
A property of type Integer could still cause this error. If you want to keep internal representation of the property as an Integer, that's fine, but change the setter so that it accepts a String, and then convert it to an Integer.