[R] data.frame with NA

[Pete]

I have this little data.frame

http://dl.dropbox.com/u/102669/nanotna.rdata

Two column contains NA, so the best thing to do is use na.locf function (with
fromLast = T)

But locf function doesn't work because NA in my data.frame are not recognized as
real NA.

Is there a way to substitute fake NA with real NA? In this case na.locf function
should work

Thank you