[R] md.pattern ('mice') failure with more than 31 variables

[saschaview at gmail.com]

Hello

How come that the function md.pattern() from package 'mice' delivers a 
warning when run over data sets with more than 31 variables?

library( 'mice' )
x <- as.data.frame(
   matrix(
     sample( c(1:3, 1:3, 1:3, NA), 7000, repl=TRUE ),
     ncol=35,
     dimnames=list(NULL,
       paste('V', 11:45, sep="")
     )
   )
)

md.pattern(x) # Warning message: In md.pattern(x) : NAs introduced by 
coercion
md.pattern(x[, 1:31]) # fine

Thanks, *S*

-- 
Sascha Vieweg, saschaview at gmail.com