using weighted.mean with tapply()

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.