绘制sf对象时出错 - 错误：找不到函数“geom_sf”

I'm currently running R Version 3.3.3 on Windows and have ggplot2 version 2.2.1 and the sf package version 0.5-4 and am getting an error saying that R could not find the function "geom_sf." I'm currently running R Version 3.3.3 on Window