I am using Struts 2.1.8 and facing validation problem in IE. I am getting the following error

An exception occurred: Error. Error message: Invalid argument. 

I tried out to figure out the cause and found the following. My generated javascript code is:

field = form.elements['district.name'];  
var error = "Enter only alphabets for district";  
if (continueValidation && field.value != null && !field.value.match("^[a-zA-Z ]*$")) {  
    addError(field, error);  
    errors = true;  
}  

I tried to mock up by putting the same code in a function and calling it in onclick event. The method addError() throws the exception and the reason is field variable. If I change it to field[0], it works fine. How to fix this error?