Making devtools an external package leads to errors in RStudio, but not in console R

Test script:

#!/bin/sh

set -x
set -e

rm -rf test
mkdir test
cd test
Rscript -e "options(repos='http://cran.rstudio.com'); packrat::init(options = list(external.packages = 'devtools', load.external.packages.on.startup = FALSE))"
echo "Version: 1.0" > test.Rproj
rm -rf packrat/lib*
R --no-save --interactive < /dev/null

After opening test.Rproj, RStudio is started with the following message on the console:

Error installing package: Error in results[sapply(results, Negate(isTRUE))] : 
  invalid subscript type 'list'
Calls: source ... afterPackratModeOn -> updateSettings -> symlinkExternalPackages
In addition: Warning messages:
1: In FUN("devtools"[[1L]], ...) :
  Package 'devtools' not available in repository or locally
2: In symlinkExternalPackages(project = project) :
  The following external packages could not be located:
- 'devtools'
Execution halted

Error installing package: Error in results[sapply(results, Negate(isTRUE))] : 
  invalid subscript type 'list'
Calls: source ... afterPackratModeOn -> updateSettings -> symlinkExternalPackages
In addition: Warning messages:
1: In FUN("devtools"[[1L]], ...) :
  Package 'devtools' not available in repository or locally
2: In symlinkExternalPackages(project = project) :
  The following external packages could not be located:
- 'devtools'
Execution halted

Then:

> library(devtools)
Error in library(devtools) : there is no package called ‘devtools’
> devtools::install
Error in loadNamespace(name) : there is no package called ‘devtools’
> packrat::on()
Packrat mode on. Using library in directory:
- "/home/muelleki/tmp/packrat-infer/test/packrat/lib"
> library(devtools)
>

Tested with current version e0d8f9b6cfa82.

Start with the reproduced shell script and the afterPackratModeOn -> updateSettings -> symlinkExternalPackages call path shown in the error. Compare external.packages handling between console R and RStudio, then verify that devtools can be loaded after opening test.Rproj without the invalid-subscript error.