diff --git a/repo2docker/buildpacks/r.py b/repo2docker/buildpacks/r.py
index 217ac0e7..9cb59f3f 100644
--- a/repo2docker/buildpacks/r.py
+++ b/repo2docker/buildpacks/r.py
@@ -344,9 +344,9 @@ class RBuildPack(PythonBuildPack):
                 "${NB_USER}",
                 # Install a pinned version of devtools, IRKernel and shiny
                 rf"""
-                EXPANDED_CRAN_MIRROR_URL="$(. /etc/os-release && echo {cran_mirror_url} | envsubst)" && \
-                R --quiet -e "install.packages(c('devtools', 'IRkernel', 'shiny'), repos='${{EXPANDED_CRAN_MIRROR_URL}}')" && \
-                R --quiet -e "IRkernel::installspec(prefix='$NB_PYTHON_PREFIX')"
+                export EXPANDED_CRAN_MIRROR_URL="$(. /etc/os-release && echo {cran_mirror_url} | envsubst)" && \
+                R --quiet -e "install.packages(c('devtools', 'IRkernel', 'shiny'), repos=Sys.getenv(\"EXPANDED_CRAN_MIRROR_URL\"))" && \
+                R --quiet -e "IRkernel::installspec(prefix=Sys.getenv(\"NB_PYTHON_PREFIX\"))"
                 """,
             ),
         ]