[R] Error: arguments imply differing number of rows

[Jeffrey T. Steedle]

I am simply trying to put 4 boxplots on the same plot, but I keep  
getting the "arguments imply differing number of rows" error. Here, I  
have two variables: a factor x of length 1762 (with 4 levels) and a  
numeric variable y of length 1762. How is it that my arguments imply  
differing numbers of rows?

> length(x)
[1] 1762
> length(y)
[1] 1762
> is.factor(x)
[1] TRUE
> is.numeric(y)
[1] TRUE
> plot(x,y)
Error in data.frame(group, x) :
   arguments imply differing number of rows: 1762, 2165
> boxplot(y~x)
Error in data.frame(group, x) :
   arguments imply differing number of rows: 1762, 2165