where are these NAs coming from?

I see this:
--8<---------------cut here---------------start------------->8---

length(which(is.na(z$language)))

[1] 0

locals <- z[z$country == mycountry,]
length(which(is.na(locals$language)))

[1] 229
--8<---------------cut here---------------end--------------->8---
where are those locals without the language coming from?!

Sam Steingold (http://sds.podval.org/) on Ubuntu 12.04 (precise) X 11.0.11103000
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