[R] using weighted.mean with tapply()

[roger bos]

I am trying to calculate the weighted mean for a of 10 deciles and I
get an error:
> decile <- tapply(X=mat$trt1m, INDEX=mat$Rank, FUN=weighted.mean, w=mat$mcap)
Error in FUN(X[[1]], ...) : 'x' and 'w' must have the same length

All three of my inputs have the same length, as shown below, and the
weighted.mean calculation works by itself, just not in tapply()

> length(mat$Rank)
[1] 1853
> length(mat$mcap)
[1] 1853
> length(mat$trt1m)
[1] 1853
> mean(mat$trt1m)
[1] -0.04475397
weighted.mean(mat$trt1m, w=mat$mcap)
[1] -0.04819243
> mat$mcap[is.na(mat$mcap)] <- min(mat$mcap, na.rm=TRUE)

I am probably making a simple error in how I pass the optional
parameter w.  Any help would be greatly appreciated.