diff --git a/server/CHANGELOG.md b/server/CHANGELOG.md
index 80f6568..8909a9f 100644
--- a/server/CHANGELOG.md
+++ b/server/CHANGELOG.md
@@ -12,6 +12,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* shell syntax error when ${btrfs_bin} variable is empty
+
 ### Security
 
 ## [22.04] - 2022-04-20
diff --git a/server/lib/bkctld-inc-lock b/server/lib/bkctld-inc-lock
index e3c704e..8b04e93 100755
--- a/server/lib/bkctld-inc-lock
+++ b/server/lib/bkctld-inc-lock
@@ -13,7 +13,7 @@ target_path="${2:?}"
 lock_target() {
     target="${1:?}"
     btrfs_bin=$(command -v btrfs)
-    if [ -z ${btrfs_bin} ]; then
+    if [ -z "${btrfs_bin}" ]; then
         error "btrfs not found. Please install btrfs-progs."
     fi
     if is_btrfs "${target}"; then
@@ -26,7 +26,7 @@ lock_target() {
 unlock_target() {
     target="${1:?}"
     btrfs_bin=$(command -v btrfs)
-    if [ -z ${btrfs_bin} ]; then
+    if [ -z "${btrfs_bin}" ]; then
         error "btrfs not found. Please install brtfs-progs."
     fi
     if is_btrfs "${target}"; then
diff --git a/server/lib/bkctld-remove b/server/lib/bkctld-remove
index f3dc6c3..382bac7 100755
--- a/server/lib/bkctld-remove
+++ b/server/lib/bkctld-remove
@@ -53,7 +53,7 @@ rm -f "${CONFDIR}/${jail_name}"
 rm -rf "$(jail_config_dir "${jail_name}")"
 
 btrfs_bin=$(command -v btrfs)
-if [ -z ${btrfs_bin} ]; then
+if [ -z "${btrfs_bin}" ]; then
     error "btrfs not found. Please install btrfs-progs."
 fi
 
diff --git a/server/lib/bkctld-rm b/server/lib/bkctld-rm
index 28caef8..00451ad 100755
--- a/server/lib/bkctld-rm
+++ b/server/lib/bkctld-rm
@@ -63,7 +63,7 @@ delete_inc_btrfs() {
     inc_path=$(inc_path "${jail_name}" "${inc_name}")
     btrfs_bin=$(command -v btrfs)
 
-    if [ -z ${btrfs_bin} ]; then
+    if [ -z "${btrfs_bin}" ]; then
         error "btrfs not found. Please install btrfs-progs."
     fi