使用带有yearmon数据的if子句时出错

library(zoo)
library(lubridate)
yearmon <- as.yearmon(c("01-10", "02-15", "03-30"), "%m-%y")
for (i in yearmon) {
if (year(yearmon[i]) > 2020) {
year(yearmon[i]) <- year(yearmon[i]) - 100
}}

Error in if (year(yearmona[i]) > 2020) { : 
missing value where TRUE/FALSE needed
library(zoo)
library(lubridate)
yearmon <- as.y