Sys.setlocale

When checking an R package of mine, I get

* checking R files for syntax errors ... WARNING
Warning in Sys.setlocale("LC_CTYPE", "en_US") :
   OS reports request to set locale to "en_US" cannot be honored

'Sys.setlocale' is not used in any of my R functions. What should I do 
to fix the Warning? I have never seen this before.

My locale is "sv_SE", on ubuntu 9.04

 > sessionInfo()
R version 2.9.1 Patched (2009-07-27 r49006)
x86_64-unknown-linux-gnu

locale:
LC_CTYPE=sv_SE;LC_NUMERIC=C;LC_TIME=sv_SE;LC_COLLATE=sv_SE;LC_MONETARY=C;LC_MESSAGES=sv_SE;LC_PAPER=sv_SE;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=sv_SE;LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

G?ran

G?ran Brostr?m               phone: 46 90 786 5223; 46 705 197 507
Department of Statistics     fax: 46 90 786 6614
Ume? University              email: gb at stat.umu.se
SE-90187 Ume?, Sweden        http://tal.stat.umu.se/~gb