Warning on assignment.

I have a question on whether a warning message is valid or if I just don't understand the process. Let me illustrate via some R code:

x <- 1:20
i <- x %% 2 > 0
y <- rep(1,20)

x[i] <- y
Warning message:
In x[i] <- y :
  number of items to replace is not a multiple of replacement length

But it still does what I would expect for the assignment:

x

[1]  1  2  1  4  1  6  1  8  1 10  1 12  1 14  1 16  1 18  1 20

What don't I understand?

Thank you.

Kevin