Error in moveInstalledPackageToCache(package)

I'm running RStudio-Server on Ubuntu 16.04 with Microsoft R Open. When I packrat::snapshot() with use.cache=TRUE, I get the following error:

> packrat::init()
Initializing packrat project in directory:
- "/ebio/abt3_projects/software/dev/tmp/test_r_package2"
Packrat mode off. Resetting library paths to:
- "/ebio/abt3/nyoungblut/R/x86_64-pc-linux-gnu-library/3.4"
- "/opt/microsoft/ropen/3.4.3/lib64/R/library"
Error in moveInstalledPackageToCache(package) : 
  no DESCRIPTION file at path '/ebio/abt3_projects/software/dev/tmp/test_r_package2/packrat/lib/x86_64-pc-linux-gnu/3.4.3/backports/DESCRIPTION' (not an R package?)
In addition: There were 44 warnings (use warnings() to see them)

However, that DESCRIPTION file does exist, with the permissions -rw-r--r--.

> packrat::get_opts()
$auto.snapshot
[1] FALSE

$use.cache
[1] TRUE

$print.banner.on.startup
[1] "auto"

$vcs.ignore.lib
[1] TRUE

$vcs.ignore.src
[1] FALSE

$external.packages
character(0)

$local.repos
character(0)

$load.external.packages.on.startup
[1] TRUE

$ignored.packages
character(0)

$quiet.package.installation
[1] TRUE

$snapshot.recommended.packages
[1] FALSE

$snapshot.fields
[1] "Imports"   "Depends"   "LinkingTo"

> sessionInfo()
R version 3.4.3 (2017-11-30)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 16.04.4 LTS

Matrix products: default
BLAS: /opt/microsoft/ropen/3.4.3/lib64/R/lib/libRblas.so
LAPACK: /opt/microsoft/ropen/3.4.3/lib64/R/lib/libRlapack.so

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

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

other attached packages:
[1] checkpoint_0.4.3     RevoUtilsMath_10.0.1

loaded via a namespace (and not attached):
 [1] Rcpp_0.12.14     packrat_0.4.8-1  digest_0.6.13    rprojroot_1.3-1  backports_1.1.2  magrittr_1.5     evaluate_0.10.1 
 [8] stringi_1.1.6    rmarkdown_1.8    RevoUtils_10.0.7 tools_3.4.3      stringr_1.2.0    yaml_2.1.16      compiler_3.4.3  
[15] htmltools_0.3.6  knitr_1.18  

Reproduce the failure with packrat::snapshot(use.cache=TRUE) in the reported RStudio Server and Ubuntu environment. Start at moveInstalledPackageToCache(package), using the reported backports/DESCRIPTION path and warnings as clues. Done means the existing DESCRIPTION file is handled correctly and snapshot completes without this error.