Error: arguments imply differing number of rows

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