trying to add Fields ( Map

I´m trying to use reflection to get all the fields from PostExper class which are defined as (Map) and add them to a similar map declared in another class any help is welcomed

While executing the next I end up with

"Method threw java.lang.IllegalArgumentException' exception."

java.lang.IllegalArgumentException: Can not set java.util.Map field EndpointRequests.LegacyEndpoints.PostExpertManagerActionRequest.param_form_id to EndpointRequests.LegacyEndpoints.PostExpertManagerActionRequest$16

suppressedExceptions = {Collections$UnmodifiableRandomAccessList@3587}  size = 0

public class PostExper {
    //attributes
    Map<String,String> header_connection=new HashMap<String,String>();
    Map<String,String> header_cacheControl=new HashMap<String,String>();
}

//method snipped used 
Class pExMAcReq = PostExpert.class;
try {
    Field[] fields = pExMAcReq.getDeclaredFields();
    for(Field f:fields){
       if(f.getName().startsWith(prefix)){
           Map<String,String> instance = new HashMap<String,String>();
           Object res = f.get(instance); //Exception instance is not writeable
           map.putAll(res);
       }
    }
}catch(Throwable e){}