Error while validating the input

I have a model Person class which holds users input data

RegisterAction class which acts as an Action is

A form page register.jsp which asks for user input

struts.xml is configured as

If I feel out all the user input form fields, everything works fine but if I keep forms "Enter age" field blank, the framework throws exception
WARNING: Error setting expression 'beanPerson.age' with value '[Ljava.lang.String;@b6434f'
ognl.MethodFailedException: Method "setAge" failed for object Register.Model.Person@2ec5b6 [java.lang.NoSuchMethodException: Register.Model.Person.setAge([Ljava.lang.String;)]
Caused by: java.lang.NoSuchMethodException: Register.Model.Person.setAge([Ljava.lang.String;)
at ognl.OgnlRuntime.callAppropriateMethod(OgnlRuntime.java:1206)

I am not able to figure it out, what is going wrong in this.

What happens when "age" is an Integer (not int) field?

Changing the type of age attribute from int to Integer and leaving the form field for age as blank now throws java.lang.NullPointerException in validate method