ReadGrib（来自rNOMADS）返回空列表

Question

urls.out <- CrawlModels(abbrev = "gfs_0p50", depth = 2)
levels <- c("2 m above ground", "800 mb")
variables <- c("TMP", "RH") #Temperature and relative humidity
grib.info <- GribGrab(urls.out[2], my.pred, levels, variables)
ReadGrib(grib.info[[1]]$file.name, variables = variables, levels = levels)

为什么ReadGrib会返回空列表？此示例非常接近ReadGrib用法的文档：https://cran.r-project.org/web/packages/rNOMADS/rNOMADS.pdf