error in if statement in r-missing value where TRUE/FALSE needed

I build this function in order to implement left_join in the "hard way". this is my code:

combined_2_df=function(df1,df2){
  my_df<-df1
  my_df$engine<-NA
  i=1
  while(i<=nrow(my_df)){
    for(j in 1: nrow(df2))
    {
      if (my_df$year[i]==df2$year[j])
      {
        my_df$engine[i]=df2$engine[j]
        break
      }#if
    }#for
    i=i+1
  }#while
  
  return(my_df)
}

however I got an error:

Error in if (my_df$year[i] == df2$year[j]) { : 
  missing value where TRUE/FALSE needed

what I do wrong? Thank you