diff --git a/tests/overlay-local-store/add-lower-inner.sh b/tests/overlay-local-store/add-lower-inner.sh
new file mode 100755
index 000000000..40a1f397c
--- /dev/null
+++ b/tests/overlay-local-store/add-lower-inner.sh
@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+
+set -eu -o pipefail
+
+set -x
+
+source common.sh
+
+# Avoid store dir being inside sandbox build-dir
+unset NIX_STORE_DIR
+unset NIX_STATE_DIR
+
+storeDirs
+
+initLowerStore
+
+mountOverlayfs
+
+# Add something to the overlay store
+overlayPath=$(addTextToStore "$storeB" "overlay-file" "Add to overlay store")
+stat "$TEST_ROOT/merged-store/$overlayPath"
+
+# Now add something to the lower store
+lowerPath=$(addTextToStore "$storeA" "lower-file" "Add to lower store")
+stat "$TEST_ROOT/store-a/$lowerPath"
+
+# Remount overlayfs to ensure synchronization
+mount -o remount "$TEST_ROOT/merged-store/nix/store"
+
+# Path should be accessible via overlay store
+stat "$TEST_ROOT/merged-store/$lowerPath"
diff --git a/tests/overlay-local-store/add-lower.sh b/tests/overlay-local-store/add-lower.sh
new file mode 100755
index 000000000..f0ac46a91
--- /dev/null
+++ b/tests/overlay-local-store/add-lower.sh
@@ -0,0 +1,5 @@
+source common.sh
+
+requireEnvironment
+setupConfig
+execUnshare ./add-lower-inner.sh
diff --git a/tests/overlay-local-store/common.sh b/tests/overlay-local-store/common.sh
index 149102000..7850c068b 100644
--- a/tests/overlay-local-store/common.sh
+++ b/tests/overlay-local-store/common.sh
@@ -8,7 +8,7 @@ requireEnvironment () {
 }
 
 setupConfig () {
-  echo "drop-supplementary-groups = false" >> "$NIX_CONF_DIR"/nix.conf
+  echo "require-drop-supplementary-groups = false" >> "$NIX_CONF_DIR"/nix.conf
   echo "build-users-group = " >> "$NIX_CONF_DIR"/nix.conf
 }
 
@@ -54,3 +54,12 @@ initLowerStore () {
 execUnshare () {
   exec unshare --mount --map-root-user "$@"
 }
+
+addTextToStore() {
+  storeDir=$1; shift
+  filename=$1; shift
+  content=$1; shift
+  filePath="$TEST_HOME/$filename"
+  echo "$content" > "$filePath"
+  nix-store --store "$storeDir" --add "$filePath"
+}
diff --git a/tests/overlay-local-store/local.mk b/tests/overlay-local-store/local.mk
index b94238a67..5d14e2561 100644
--- a/tests/overlay-local-store/local.mk
+++ b/tests/overlay-local-store/local.mk
@@ -2,6 +2,7 @@ overlay-local-store-tests := \
   $(d)/check-post-init.sh \
   $(d)/redundant-add.sh \
   $(d)/build.sh \
-  $(d)/bad-uris.sh
+  $(d)/bad-uris.sh \
+  $(d)/add-lower.sh
 
 install-tests-groups += overlay-local-store
diff --git a/tests/overlay-local-store/redundant-add-inner.sh b/tests/overlay-local-store/redundant-add-inner.sh
index 97969b40e..cfdae68b4 100755
--- a/tests/overlay-local-store/redundant-add-inner.sh
+++ b/tests/overlay-local-store/redundant-add-inner.sh
@@ -7,7 +7,7 @@ set -x
 source common.sh
 
 # Avoid store dir being inside sandbox build-dir
-unset NIX_STORE_DIR
+unset NIX_STORE_DIR  # TODO: This causes toRealPath to fail (it expects this var to be set)
 unset NIX_STATE_DIR
 
 storeDirs
@@ -27,4 +27,5 @@ path=$(nix-store --store "$storeB" --add ../dummy)
 stat $(toRealPath "$storeA/nix/store" "$path")
 
 # upper layer should still not have it (no redundant copy)
-expect 1 stat $(toRealPath "$storeB/nix/store" "$path")
+expect 1 stat $(toRealPath "$storeB/nix/store" "$path")  # TODO: Check this is failing for the right reason.
+                                                         # $storeB is a store URI not a directory path