diff --git a/src/r-apt/NOTES.md b/src/r-apt/NOTES.md
index c01382d..190cb25 100644
--- a/src/r-apt/NOTES.md
+++ b/src/r-apt/NOTES.md
@@ -6,6 +6,8 @@
 
 If the `useTesting` is `true`, `linux/arm64` platform `debian` also supported.
 
+Note that this Feature only supports non-R images. If R is already installed, installation will be failed.
+
 ## R package installation
 
 ### Binary installation via apt
diff --git a/src/r-apt/devcontainer-feature.json b/src/r-apt/devcontainer-feature.json
index d78074f..5ad13b0 100644
--- a/src/r-apt/devcontainer-feature.json
+++ b/src/r-apt/devcontainer-feature.json
@@ -1,7 +1,7 @@
 {
 	"name": "R (via apt)",
 	"id": "r-apt",
-	"version": "0.4.0",
+	"version": "0.4.1",
 	"description": "Installs the latest R, some R packages, and needed dependencies. Note: May require source code compilation for some R packages.",
 	"documentationURL": "https://github.com/rocker-org/devcontainer-features/tree/main/src/r-apt",
 	"options": {
diff --git a/src/r-apt/install.sh b/src/r-apt/install.sh
index 5747e1c..b4099a4 100755
--- a/src/r-apt/install.sh
+++ b/src/r-apt/install.sh
@@ -18,6 +18,12 @@ PIP_PACKAGES=()
 
 set -e
 
+if R --version >/dev/null 2>&1 && dpkg -s r-base >/dev/null 2>&1; then
+    echo "(!) R has already been installed via a non-apt method."
+    echo "    This script is designed only for non-R Debian and Ubuntu."
+    exit 1
+fi
+
 # Clean up
 rm -rf /var/lib/apt/lists/*
 
@@ -156,8 +162,8 @@ elif [ "${ID}" = "debian" ]; then
     if [ "${USE_TESTING}" = "true" ]; then
         echo "Set up Debian testing..."
         echo "deb http://http.debian.net/debian testing main" >/etc/apt/sources.list.d/debian-testing.list
+        echo "deb http://http.debian.net/debian-security testing-security main" >>/etc/apt/sources.list.d/debian-testing.list
         echo "deb http://http.debian.net/debian unstable main" >/etc/apt/sources.list.d/debian-unstable.list
-        echo 'APT::Default-Release "testing";' >/etc/apt/apt.conf.d/default
     else
         echo "Set up for Debian ${VERSION_CODENAME}..."
         curl -fsSL "https://keyserver.ubuntu.com/pks/lookup?op=get&search=0x95c0faf38db3ccad0c080a7bdc78b2ddeabc47b7" | tee -a /etc/apt/trusted.gpg.d/cran_debian_key.asc >/dev/null
@@ -218,7 +224,7 @@ fi
 if [ "${ID}" = "debian" ] && [ "${install_httpgd}" = "true" ]; then
     check_packages \
         g++ make r-cran-later r-cran-systemfonts r-cran-bh \
-        libxml2-dev libicu-dev libcairo2-dev libfontconfig1-dev libfreetype6-dev libpng-dev
+        libxml2-dev libicu-dev libcairo2-dev libfontconfig1-dev libfreetype-dev libpng-dev
     R -q -e 'install.packages("httpgd")'
 fi