[R] package zoo, function na.spline with option maxgap -> Error: attempt to apply non-function?

[May Katharina]

Hello,

I'm trying to use na.spline (package zoo) to fill some missing data in a time series.
this works fine, however, if I apply the 'maxgap' argument, I always get the error:
<------
Error in na.spline.vec(x., coredata(object.), xout = xout., ...) :  attempt to apply non-function
------>

I couldn't find a similar error for this case in the mailing lists and zoo vignette, I hope I didn't
miss any basic thing, but as I don't know how to proceed, any help on why this isn't working
is very much appreciated  (I guess I don't have to mention I'm new to time series & zoo and that I'm terrible sorry if this is
an absolute dull question). 

The data I use is rather large, but the error can be reconstructed in a small example (s. below). 
If of any interest: I use R 2.15.0 GUI 1.51 Leopard build 32-bit (6148).

Many thanks and best regards, Katharina


<------
library(zoo)

#create data + gaps
z <- zoo(seq(1:30), seq(1900, length =30 )) 
z[c(2,5,6,7,9,10,15,16,17,20,21)] <- NA

> z
1900 1901 1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 
   1   NA    3    4   NA   NA   NA    8   NA   NA   11   12   13   14   NA   NA   NA   18   19   NA   NA   22 
1922 1923 1924 1925 1926 1927 1928 1929 
  23   24   25   26   27   28   29   30 

#1. fill using spline: this version without gap restriction works
> na.spline(z)
1900 1901 1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 
   1    2    3    4    5    6    7    8    9   10   11   12   13   14   15   16   17   18   19   20   21   22 
1922 1923 1924 1925 1926 1927 1928 1929 
  23   24   25   26   27   28   29   30 

#2. fill using spline: a=with applying 'maxgap' an error is produced
> na.spline(z, maxgap=2)
Error in na.spline.vec(x., coredata(object.), xout = xout., ...) : 
  attempt to apply non-function
------>