diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6666e72..2da55f7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+- The cache is now correctly invalidated if the stack version of an existing cache cannot be determined. ([#133](https://github.com/heroku/heroku-buildpack-apt/pull/133))
+- When the cache is invalidated, all APT-related files are now removed, rather than only some of them. In particular, this means old APT package index files are cleaned up. ([#133](https://github.com/heroku/heroku-buildpack-apt/pull/133))
+
 ## 2024-03-28
 
 - Warn when Aptfile contains no packages ([#126](https://github.com/heroku/heroku-buildpack-apt/pull/126))
diff --git a/bin/compile b/bin/compile
index 711690a..ac34854 100755
--- a/bin/compile
+++ b/bin/compile
@@ -31,7 +31,7 @@ function indent() {
 
 if ! grep --invert-match -e "^\s*#" -e "^\s*$" -e "^:repo:" -q "${BUILD_DIR}/Aptfile"; then
   echo "
-!       You have no packages listed in your Aptfile. If you don't need custom Apt packages,
+!       You have no packages listed in your Aptfile. If you don't need custom APT packages,
 !       delete your Aptfile and remove the buildpack with:
 !
 !       $ heroku buildpacks:remove heroku-community/apt
@@ -40,15 +40,20 @@ if ! grep --invert-match -e "^\s*#" -e "^\s*$" -e "^:repo:" -q "${BUILD_DIR}/Apt
 fi
 
 # Store which STACK we are running on in the cache to bust the cache if it changes
-if [[ -f "$CACHE_DIR/.apt/STACK" ]]; then
-  CACHED_STACK=$(cat "$CACHE_DIR/.apt/STACK")
+# This file really should be inside "${CACHE_DIR}/apt" not "${CACHE_DIR}/.apt", however, this
+# buildpack has used the wrong directory name for it for some time, so there are many APT-related
+# forks that use this path, so for backwards compatibility we're not changing the path, so as to
+# not affect apps that have multiple APT-using buildpacks active at the same time.
+STACK_VERSION_FILE="${CACHE_DIR}/.apt/STACK"
+if [[ -f "${STACK_VERSION_FILE}" ]]; then
+  CACHED_STACK=$(cat "${STACK_VERSION_FILE}")
 else
-  CACHED_STACK=$STACK
+  CACHED_STACK=
 fi
 
 # Ensure we store the STACK in the cache for next time.
-mkdir -p "$CACHE_DIR/.apt"
-echo "$STACK" > "$CACHE_DIR/.apt/STACK"
+mkdir -p "${CACHE_DIR}/.apt"
+echo "${STACK}" > "${STACK_VERSION_FILE}"
 
 APT_CACHE_DIR="$CACHE_DIR/apt/cache"
 APT_STATE_DIR="$CACHE_DIR/apt/state"
@@ -70,7 +75,7 @@ if [[ -f "$APT_CACHE_DIR/Aptfile" ]] && cmp -s "$BUILD_DIR/Aptfile" "$APT_CACHE_
 else
   # Aptfile changed or does not exist or STACK changed
   topic "Detected Aptfile or Stack changes, flushing cache"
-  rm -rf "$APT_CACHE_DIR"
+  rm -rf "${CACHE_DIR}/apt"
   mkdir -p "$APT_CACHE_DIR/archives/partial"
   mkdir -p "$APT_STATE_DIR/lists/partial"
   mkdir -p "$APT_SOURCELIST_DIR"   # make dir for sources
@@ -89,7 +94,7 @@ APT_OPTIONS=("-o" "debug::nolocking=true" "-o" "dir::cache=$APT_CACHE_DIR" "-o"
 # Override the use of /etc/apt/sources.list (sourcelist) and /etc/apt/sources.list.d/* (sourceparts).
 APT_OPTIONS+=("-o" "dir::etc::sourcelist=$APT_SOURCES" "-o" "dir::etc::sourceparts=$APT_SOURCEPARTS_DIR")
 
-topic "Updating apt caches"
+topic "Updating APT package index"
 apt-get "${APT_OPTIONS[@]}" update 2>&1 | indent
 
 while IFS= read -r PACKAGE; do
diff --git a/test/run b/test/run
index 243237e..5c91f64 100755
--- a/test/run
+++ b/test/run
@@ -2,7 +2,7 @@
 
 testCompilePackageNames() {
   compile "package-names"
-  assertCaptured "Updating apt caches"
+  assertCaptured "Updating APT package index"
   assertCaptured "Fetching .debs for xmlsec1"
   assertCaptured "Fetching .debs for s3cmd wget"
   assertCaptured "Fetching .debs for mysql-client-*"
@@ -32,7 +32,7 @@ testCompileCustomPackageUrl() {
     [heroku-24]="https://github.com/wkhtmltopdf/packaging/releases/download/0.12.6.1-2/wkhtmltox_0.12.6.1-2.jammy_amd64.deb"
   )
   compile "custom-package-url-$STACK"
-  assertCaptured "Updating apt caches"
+  assertCaptured "Updating APT package index"
   assertCaptured "Fetching ${download_urls[$STACK]}"
   assertCaptured "Installing wkhtmltox"
   assertCaptured "Writing profile script"
@@ -62,7 +62,7 @@ testCompileCustomRepository() {
   )
   compile "custom-repository-$STACK"
   assertCaptured "Adding custom repositories"
-  assertCaptured "Updating apt caches"
+  assertCaptured "Updating APT package index"
   assertCaptured "http://us.archive.ubuntu.com/ubuntu ${ubuntu_release_names[$STACK]}/multiverse amd64 Packages"
   assertCaptured "Fetching .debs for fasttracker2"
   assertCaptured "Installing fasttracker2"
@@ -87,7 +87,7 @@ testReportCustomRepository() {
 testCompileEmpty() {
   compile "empty"
   assertCaptured "You have no packages listed in your Aptfile"
-  assertNotCaptured "Updating apt caches"
+  assertNotCaptured "Updating APT package index"
   assertCapturedSuccess
 }
 
@@ -102,7 +102,7 @@ testReportEmpty() {
 testCompileOnlyComments() {
   compile "only-comments"
   assertCaptured "You have no packages listed in your Aptfile"
-  assertNotCaptured "Updating apt caches"
+  assertNotCaptured "Updating APT package index"
   assertCapturedSuccess
 }
 
@@ -117,7 +117,7 @@ testReportOnlyComments() {
 testCompileCustomRepositoryNoPackages() {
   compile "custom-repository-no-packages"
   assertCaptured "You have no packages listed in your Aptfile"
-  assertNotCaptured "Updating apt caches"
+  assertNotCaptured "Updating APT package index"
   assertCapturedSuccess
 }