[R-SIG-Mac] Sys.timezone == NA?

[BATES Timothy]

Hi,
On R 3.40, I've been getting some Warnings about timezone, e.g.

Warning message: In as.POSIXlt.POSIXct(x, tz) : unknown timezone 'default/Europe/London'

But also Sys.timezone returns NA.... Is this a bug? was there a change in tz moving to 3.4.0?
 
Sys.timezone()
[1] NA

R version: R version 3.4.0 Patched (2017-05-02 r72651)
Platform: x86_64-apple-darwin15.6.0 
MacOS: 10.13

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