diff --git a/tests/zfs-tests/include/libtest.shlib b/tests/zfs-tests/include/libtest.shlib
index 6b0f8b18c4b6..8b30b9b91641 100644
--- a/tests/zfs-tests/include/libtest.shlib
+++ b/tests/zfs-tests/include/libtest.shlib
@@ -1085,7 +1085,7 @@ function fill_fs # destdir dirnum filenum bytes num_writes data
 	typeset -i filenum=${3:-50}
 	typeset -i bytes=${4:-8192}
 	typeset -i num_writes=${5:-10240}
-	typeset data=${6:-0}
+	typeset data=${6:-"R"}
 
 	mkdir -p $destdir/{1..$dirnum}
 	for f in $destdir/{1..$dirnum}/$TESTFILE{1..$filenum}; do
diff --git a/tests/zfs-tests/tests/functional/fault/auto_replace_001_pos.ksh b/tests/zfs-tests/tests/functional/fault/auto_replace_001_pos.ksh
index ef49a5d50f6c..45848bec1f5a 100755
--- a/tests/zfs-tests/tests/functional/fault/auto_replace_001_pos.ksh
+++ b/tests/zfs-tests/tests/functional/fault/auto_replace_001_pos.ksh
@@ -86,7 +86,7 @@ log_must zpool set autoreplace=on $TESTPOOL
 
 # Add some data to the pool
 log_must zfs create $TESTPOOL/fs
-log_must fill_fs /$TESTPOOL/fs 4 100 4096 512 Z
+log_must fill_fs /$TESTPOOL/fs 4 100 4096 512 R
 log_must zpool export $TESTPOOL
 
 # Record the partition UUID for later comparison
diff --git a/tests/zfs-tests/tests/functional/fault/auto_replace_002_pos.ksh b/tests/zfs-tests/tests/functional/fault/auto_replace_002_pos.ksh
index a77957f32255..878b4e450340 100755
--- a/tests/zfs-tests/tests/functional/fault/auto_replace_002_pos.ksh
+++ b/tests/zfs-tests/tests/functional/fault/auto_replace_002_pos.ksh
@@ -119,7 +119,7 @@ log_must zpool set autoreplace=on $TESTPOOL
 
 # Add some data to the pool
 log_must zfs create $TESTPOOL/fs
-log_must fill_fs /$TESTPOOL/fs 4 100 4096 512 Z
+log_must fill_fs /$TESTPOOL/fs 4 100 4096 512 R
 log_must zpool export $TESTPOOL
 
 # Record the partition UUID for later comparison
diff --git a/tests/zfs-tests/tests/functional/fault/suspend_on_probe_errors.ksh b/tests/zfs-tests/tests/functional/fault/suspend_on_probe_errors.ksh
index 340994bb60c5..b5df1c7e37f8 100755
--- a/tests/zfs-tests/tests/functional/fault/suspend_on_probe_errors.ksh
+++ b/tests/zfs-tests/tests/functional/fault/suspend_on_probe_errors.ksh
@@ -101,7 +101,7 @@ sync_pool $TESTPOOL
 log_must zfs create $TESTPOOL/fs
 MNTPOINT="$(get_prop mountpoint $TESTPOOL/fs)"
 SECONDS=0
-log_must fill_fs $MNTPOINT 1 200 4096 10 Z
+log_must fill_fs $MNTPOINT 1 200 4096 10 R
 log_note "fill_fs took $SECONDS seconds"
 sync_pool $TESTPOOL