Inconsistent behaviour when manipulating a ts object with frequency = 12 or 4

Dear All,

I found the following  under R 2.3.0 on WINXP (tested on 2 PCs, I do
not have access to Linux from this PC, sorry ... )

set.seed(10)
x <- ts(rnorm(6),frequency=7)
x

Time Series:
Start = c(1, 1)
End = c(1, 6)
Frequency = 7
[1]  0.01874617 -0.18425254 -1.37133055 -0.59916772  0.29454513  0.38979430

x[24] <- NA
x

Time Series:
Start = c(1, 1)
End = c(1, 6)
Frequency = 7
[1]  0.01874617 -0.18425254 -1.37133055 -0.59916772  0.29454513
0.38979430          NA          NA          NA          NA          NA
[12]          NA          NA          NA          NA          NA
  NA          NA          NA          NA          NA          NA
[23]
## OK ********************

x <- ts(rnorm(6),frequency=12) # THE SAME BUT WITH frequency = 12
x

        Jan        Feb        Mar        Apr        May        Jun
1 -1.2080762 -0.3636760 -1.6266727 -0.2564784  1.1017795  0.7557815

x[24] <- NA
x

Error in matrix(c(rep.int("", start.pad), format(x, ...), rep.int("",  :
       length of 'dimnames' [1] not equal to array extent
In addition: Warning message:
data length [30] is not a sub-multiple or multiple of the number of
columns [12] in matrix

x <- ts(rnorm(6),frequency=4) # THE SAME BUT WITH frequency = 4
x

        Qtr1        Qtr2        Qtr3        Qtr4
1 -0.23823356  0.98744470  0.74139013  0.08934727
2 -0.95494386 -0.19515038

x[24] <- NA
x

Error in matrix(c(rep.int("", start.pad), format(x, ...), rep.int("",  :
       length of 'dimnames' [1] not equal to array extent
In addition: Warning message:
data length [26] is not a sub-multiple or multiple of the number of
rows [7] in matrix


Is this behaviour expected for frequencies 12 and 4?

Thank you

Simone

***************************************************
platform       i386-pc-mingw32
arch           i386
os             mingw32
system         i386, mingw32
status
major          2
minor          3.0
year           2006
month          04
day            24
svn rev        37909
language       R
version.string Version 2.3.0 (2006-04-24)

Sys.getlocale()

[1] "LC_COLLATE=English_United States.1252;LC_CTYPE=English_United
States.1252;LC_MONETARY=English_United
States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252"