使用setMethod的警告消息

Question

如果我是第一次运行以下代码，我会收到有关set方法的警告：

Warning message:
In getPackageName(environment(fdef)) :
  Create package name, ‘2013-09-18 09:29:59’, if nothing there

B.getFields<-function(keys){
  vars<-mget(names(.refClassDef@fieldClasses), envir = attr(.self, ".xData"))
  return(vars[keys])
}

B<-setRefClass(Class = "B"
              ,fields = list(var1 = "character")
              ,methods = list(getFields=B.getFields
                               ,initialize=function(...) {
                                   usingMethods("getFields")
                                   callSuper(...)
                                 }
                               )
)  
setGeneric("getFields", function(object, ...) standardGeneric("getFields"))
setMethod(getFields, "list", function(object, ...) lapply(object,getFields,...))