Struts validation problem with string array for text fields

Hi,
I have a requirement for struts validation for a number of text fields which will use the same name. As the number can be up to 50 text boxes, obviously, I want to use the same name.
The name of my text box if firstName[]

Below is my form beans:
package strutspackage;

import org.apache.struts.validator.ValidatorForm;

public class appForm extends ValidatorForm {

      private static final long serialVersionUID = 1L;

      private String[] firstName;
      
      public String[] getFirstName() {
            return this.firstName;
      }
      public void setFirstName(String[] firstName) {
            this.firstName = firstName;
      }
}

My validation is a simple 'required' validation:
 <field
                property="firstName"
                depends="mask"
                page="1">
                     <arg key="genericInvalid.amount"/>
                     <var>
                          <var-name>mask</var-name>
                          <var-value>^[0-9]*$</var-value>
                     </var>
            </field>

However, when I run this application form, I'm getting errors:
javax.servlet.ServletException: Invalid argument looking up property firstName[] of bean org.apache.struts.taglib.html.BEAN
      org.apache.jasper.runtime.PageContextImpl.doHandlePageException(PageContextImpl.java:858)
      org.apache.jasper.runtime.PageContextImpl.handlePageException(PageContextImpl.java:791)
      org.apache.jsp.appForm1_jsp._jspService(appForm1_jsp.java:172)
      org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
      javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
      org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:332)
      org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
      org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
      javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

I presume this has something to do with my form class.
I don't have much experience using string array within Struts so if anyone can lend a hand or has previous experience with this, it would be much appreciated.