[R] problem with svyby and NAs (survey package)

[A.F.Fenton at lse.ac.uk]

Hello

I'm trying to get the proportion "true" for dichotomous variable for
various subgroups in a survey.

This works fine, but obviously doesn't give proportions directly:
svytable(~SurvYear+problem.vandal, seh.dsn, round=TRUE)
        problem.vandal
SurvYear FALSE  TRUE
    1995  8906   786
    1997 17164  2494
    1998 17890  1921
    1999 18322  1669
    2001 17623  2122
...

Note some years are missing - they are part of the dataset, but all
responses are NA (the question wasn't asked).

However, this gives an error, and I'd like to understand why - it works
for variables without missing years:

svyby(~problem.vandal, ~SurvYear, seh.dsn, svymean, na.rm=TRUE)
Error in tapply(1:NROW(x), list(factor(strata)), function(index) { : 
  arguments must have same length

The error only occurs when na.rm=TRUE and there are no observations in
one year.

Thanks
alex

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