Error while serializing object to json Jackson

I need a json output in the format

{
main:[
{
  "tag1" : "value1",
  "tag2" :"value2",
  "tag3" :[
      {
        "subTag1" :"value3",
        "subTag2" :"value4",
        "subTag3" :"value5
      },
      {
        "subTag1" :"value3",
        "subTag2" :"value4",
      },
      {
        "subTag1" :"value3",
        "subTag3" :"value5
      }
   ]
},
 {
  "tag1" : "value1",
  "tag2" :"value2",
  "tag3" :[
      {
        "subTag1" :"value3",
        "subTag2" :"value4",
        "subTag3" :"value5
      },
      {
        "subTag1" :"value3",
        "subTag2" :"value4",
      },
      {
        "subTag1" :"value3",
        "subTag3" :"value5
      }
   ]
}
]
 }

My java class has objects of the form

public class MainObject{
   Map<String,Object> tagValueMap = new HashMap<String,Object>();
}

I would like to keep the MainObject class generic. This implementation is throwing an error when I try to create json using Jackson in Spring. An error is thrown by jackson ObjectMapper called from

 org.springframework.http.converter.json.MappingJacksonHttpMessageConverter 

writeInternal() method which says

 org.codehaus.jackson.map.JsonMappingException:
 java.util.LinkedHashMap cannot be cast to java.lang.String 

Why is it trying to cast the object to a string rather than reading it as hashmap? How can I avoid this error?

I am aware of specifically mentioning each field in the MainObject class for serialising an object. But How can I achieve the same with a generic class which will accept both string elements and array elements in the json object.

Thanks