Error when using Snakemake variable in R script

I have got the following Snakemake rule:

rule deseq2:
    input:
        salmon=expand("salmon/{sample}/quant.sf", sample=SAMPLES)
    output:
        xlsx="deseq2/deseq2_diff_trx.xlsx",
        rdata="deseq2/dds.Rdata",
    params:
        map_with,
        genome,
        gtf,
    log:
        "logs/deseq2/deseq2.log"
    conda:
        "envs/deseq2.yml"
    threads: config["resources"]["deseq2"]["cpu"]
    resources: 
        runtime=config["resources"]["deseq2"]["time"]
    script:
        "scripts/deseq2.R"

The part of that R script that the rule runs that gives an error is when I save a variable to a file name in the output section of the snakemake variable:

save(dds, snakemake@output[[2]])

Error in save(dds, snakemake@output[[2]]) : 
  object ‘snakemake@output[[2]]’ not found

Earlier in the script I access the params part of the snakemake variable in a similar way, but without any errors:

map.with <- snakemake@params[[1]]
genome <- snakemake@params[[2]]
gtf <- snakemake@params[[3]]

When I print the snakemake variable I get this (only partial,relevant output):

An object of class "Snakemake"
Slot "input":
[[1]]
[1] "salmon/Control-1/quant.sf"

[[2]]
[1] "salmon/Control-2/quant.sf"

[[3]]
[1] "salmon/Control-Hypoxia-1/quant.sf"

[[4]]
[1] "salmon/Control-Hypoxia-2/quant.sf"

$salmon
[1] "salmon/Control-1/quant.sf"         "salmon/Control-2/quant.sf"        
[3] "salmon/Control-Hypoxia-1/quant.sf" "salmon/Control-Hypoxia-2/quant.sf"


Slot "output":
[[1]]
[1] "deseq2/deseq2_diff_trx.xlsx"

[[2]]
[1] "deseq2/dds.Rdata"

$xlsx
[1] "deseq2/deseq2_diff_trx.xlsx"

$rdata
[1] "deseq2/dds.Rdata"


Slot "params":
[[1]]
[1] "salmon"

[[2]]
[1] "hg38"

[[3]]
[1] "/home/user/Documents/references/gtf/hg38/gencode.v43.annotation.gtf"

I have also saved the work space to a file for debugging as suggest by the snakemake web site. When I load this into R I can do the following things:

> snakemake@output[["rdata"]]
[1] "deseq2/dds.Rdata"

> snakemake@output[[2]]
[1] "deseq2/dds.Rdata"

But when the above code is include in the proper script I get the object not found (see above) error.

What am I doing wrong?