object instanceof MyObject[] causes ClassDefNotFound exception within Closure

public Map<String, Object> toMap() {
        Map<String, Object> params = getPropertiesWithoutClassData()
        Map<String, Object> params = new HashMap<String,Object>();
        
        params.each { 
           if(it.value instanceof MyObject[]){ 
params.add(it.key, it.value)
 }else{
params.add("test","test")
           } 
        }

   return params
}

public Map<String, Object> toMap() {
        Map<String, Object> params = getPropertiesWithoutClassData()
        Map<String, Object> params = new HashMap<String,Object>();

for(String param : params)
{
 if(params.get(param) instanceof MyObject[]){ 
params.add("works", "works")
 }else{
params.add("test","test")
           } 
}

   return params
}