Как избежать странной ошибки умлаута при использовании data.table

Мне нужно оперировать суммами в разреженном фрейме данных с учетом идентификаторов

require(data.table)
sentEx = structure(list(abend = c(1, 1, 0, 0, 2), aber = c(0, 1, 0, 0, 
0), über = c(1, 0, 0, 0, 0), überall = c(0, 0, 0, 0, 0), überlegt = c(0, 
0, 0, 0, 0), ID = structure(c(1L, 1L, 2L, 2L, 2L), .Label = c("0019", 
"0021"), class = "factor"), abgeandert = c(1, 1, 1, 0, 0), abgebildet = c(0, 
0, 1, 1, 0), abgelegt = c(0, 0, 0, 0, 3)), .Names = c("abend", 
"aber", "über", "überall", "überlegt", "ID", "abgeandert", "abgebildet", 
"abgelegt"), row.names = c(1L, 2L, 16L, 17L, 18L), class = "data.frame")

sentEx  # How it looks
   abend aber über überall überlegt   ID abgeandert abgebildet abgelegt
1      1    0    1       0        0 0019          1          0        0
2      1    1    0       0        0 0019          1          0        0
16     0    0    0       0        0 0021          1          1        0
17     0    0    0       0        0 0021          0          1        0
18     2    0    0       0        0 0021          0          0        3

Без "умлаута" работает нормально:

sentEx.dt <- data.table(sentEx[,-c(3,4,5)])[, lapply(.SD, sum), by=ID]
(sentExSum <- as.data.frame(sentEx.dt))  # Need again as dataframe, which looks like:
    ID abend aber abgeandert abgebildet abgelegt
1 0019     2    1          2          0        0
2 0021     2    0          1          2        3 

Но в противном случае я получаю эту ошибку:

sentEx.dt <- data.table(sentEx)[, lapply(.SD, sum), by=ID]
# Error in gsum(`über`) : object 'über' not found
      sentExSum <- as.data.frame(sentEx.dt)

Некоторая дополнительная информация о сеансе (поскольку проблема связана с системой - см. комментарии):

sessionInfo()
R version 3.0.2 (2013-09-25)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252    LC_MONETARY=German_Germany.1252 LC_NUMERIC=C                   
[5] LC_TIME=German_Germany.1252    

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

other attached packages:
[1] data.table_1.9.2

loaded via a namespace (and not attached):
[1] plyr_1.8.1     Rcpp_0.11.0    reshape2_1.2.2 stringr_0.6.2  tools_3.0.2

Также запрашиваются команды:

require(data.table); test.data.table()
Running C:/Users/Krohana/Documents/R/win-library/3.0/data.table/tests/tests.Rraw 
Loading required package: reshape
Loading required package: hexbin
Loading required package: xts
Loading required package: bit64
Test 167.2 not run. If required call library(hexbin) first.
Don't know how to automatically pick scale for object of type ITime. Defaulting to continuous
Don't know how to automatically pick scale for object of type ITime. Defaulting to continuous
Tests 487 and 488 not run. If required call library(reshape) first.
Test 841 not run. If required call library(xts) first.
Tests 897-899 not run. If required call library(bit64) first.
All 1220 tests in inst/tests/tests.Rraw completed ok in 24.321sec on Sun Mar 02 17:57:26 2014 ts/tests.Rraw completed ok in 24.638sec on Sun Mar 02 17:55:45 2014

Запрошенные команды2:

> Encoding(names(sentEx))
[1] "unknown" "unknown" "UTF-8"   "UTF-8"   "UTF-8"   "unknown" "unknown" "unknown" "unknown"
> options(datatable.verbose=TRUE)
> options(datatable.verbose=TRUE); options(datatable.optimize=1L);