replacing elements of a zoo object

Dear R Users,

I am trying to do something quite simple: replace the elements of a zoo
object. For some reason, the following code does not seem to work. How can
I replace the value for the 14th of Dec of 2008 in the zoo object x below
with 1 (it is currently NA).

x

2008-12-11 2008-12-12 2008-12-13 2008-12-14 2008-12-15 2008-12-16
  361.667    389.875         NA         NA    397.822    395.667

class(x)

[1] "zoo"

class(index(x))

[1] "Date"

x[as.Date("2008-12-14"),]

2008-12-14
       NA

x[as.Date("2008-12-14"),]<-1

Error in x[as.Date("2008-12-14"), ] <- 1 :
 incorrect number of subscripts on matrix

Thanks in advance,
Tolga

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