scale_fill_manual define color for NA values

I try to make a barplot with ggplot2 and am facing some issues with defining the color for NA.

ggh <- ggplot(data=dat, aes(x=var1, fill=var2))+
  geom_bar(position="dodge")+
  scale_fill_manual(
    values=c("s"="steelblue", "i"="darkgoldenrod2", "r"="firebrick4", na.value="black"))

In my var2 I have values c("s", "i", "r", NA). For some reason my code above inside the scale_fill_manual does not work for NA, even if it works fine for all the others values.

Can someone help me figure out why?

Thanks for the help