diff --git a/scripts/pct-shrink-rootfs b/scripts/pct-shrink-rootfs
index d4aa48b..f054fcd 100755
--- a/scripts/pct-shrink-rootfs
+++ b/scripts/pct-shrink-rootfs
@@ -133,7 +133,7 @@ main() {
   mounted=1
   pct mount "$ctid"
   used=$(df -B1 --output=used "$root" | tail -n1 | tr -d ' ')
-  iused=$(df -i --output=iused "$root" | tail -n1 | tr -d ' ')
+  iused=$(df --output=iused "$root" | tail -n1 | tr -d ' ')
   unmount_or_die
   [[ $used =~ ^[0-9]+$ && $iused =~ ^[0-9]+$ ]] || die "cannot read df output"
 
diff --git a/tests/test_pct_shrink.py b/tests/test_pct_shrink.py
index 1fc6622..3b6329a 100644
--- a/tests/test_pct_shrink.py
+++ b/tests/test_pct_shrink.py
@@ -20,7 +20,9 @@ STUBS = {
     "ha-manager": 'echo "${STUB_HA_OUT:-}"',
     "pvesr": 'echo "${STUB_PVESR_OUT:-}"',
     "df": """echo "df $*" >> "$STUB_LOG"
-if [[ "$*" == *-i* ]]; then echo IUsed; echo "${STUB_IUSED:-1000}"
+# like GNU df: -i and --output cannot be combined
+if [[ " $* " == *" -i "* && "$*" == *--output* ]]; then echo "df: options -i and --output are mutually exclusive" >&2; exit 1; fi
+if [[ "$*" == *--output=iused* ]]; then echo IUsed; echo "${STUB_IUSED:-1000}"
 else echo Used; echo "${STUB_USED:-1073741824}"; fi""",
     "rsync": """echo "rsync $*" >> "$STUB_LOG"
 [[ -z ${STUB_RSYNC_FAIL:-} ]]""",