Standard non-standard evaluation problem with 2.10-0

Dear List

I am getting an error when checking my analogue package with
R2.10.0-patched. The error comes when running a function within which I
use the standard non-standard evaluation method. I've distilled the
error and functions involved out into the following simple example to
illustrate the error:

## Dummy data to illustrate formula method
d <- data.frame(A = runif(10), B = runif(10), C = runif(10))
## simulate some missings
d[sample(10,3), 1] <- NA

foo <- function(formula, data = NULL,
                subset = NULL,
                na.action = na.pass, ...) {
    mf <- match.call()
    mf[[1]] <- as.name("model.frame")
    mt <- terms(formula, data = data, simplify = TRUE)
    mf[[2]] <- formula(mt, data = data)
    mf$na.action <- substitute(na.action)
    dots <- list(...)
    mf[[names(dots)]] <- NULL
    mf <- eval(mf,parent.frame())
    mf
}

## apply foo using formula
foo(~ . - B, data = d, na.action = na.pass,
    method = "missing", na.value = 0)
Error in mf[[names(dots)]] <- NULL : 
  more elements supplied than there are to replace

If I debug(foo) and do:

Browse[2]> names(dots)
[1] "method"   "na.value"
Browse[2]> names(mf)
[1] ""          "formula"   "data"      "na.action" "method"   
[6] "na.value"
Browse[2]> mf[[names(dots)[1]]]
[1] "missing"
Browse[2]> mf[[names(dots)[2]]]
[1] 0

But
Browse[2]> mf[[names(dots)]]
Error in mf[[names(dots)]] : subscript out of bounds
Browse[2]> str(names(dots))
 chr [1:2] "method" "na.value"

I could have sworn I tested this during the beta test phase for 2.10.0 -
if I did I didn't get any errors at that time - and this code works fine
under R2.9.x branch. The package is now failing checks on CRAN and on my
local install.

Am I doing something patently stupid here? Has something changed in '[['
or 'names' that I'm now running foul of? I can probably work round this
by setting the individual names of 'mf' to NULL in two calls, but I'd
like to get to the bottom of the problem if at all possible.

Session Info:
R version 2.10.0 Patched (2009-11-01 r50276) 
i686-pc-linux-gnu 

locale:
 [1] LC_CTYPE=en_GB.UTF-8       LC_NUMERIC=C              
 [3] LC_TIME=en_GB.UTF-8        LC_COLLATE=en_GB.UTF-8    
 [5] LC_MONETARY=C              LC_MESSAGES=en_GB.UTF-8   
 [7] LC_PAPER=en_GB.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=en_GB.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods  
[7] base     

other attached packages:
[1] analogue_0.6-21 MASS_7.2-48     lattice_0.17-25
[4] vegan_1.15-3   

loaded via a namespace (and not attached):
[1] grid_2.10.0  tools_2.10.0

Thanks in advance,

G

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 Dr. Gavin Simpson             [t] +44 (0)20 7679 0522
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