diff --git a/tests/tests/basic-truncate.sh b/tests/tests/basic-truncate.sh
index d7479640..c7a3906f 100644
--- a/tests/tests/basic-truncate.sh
+++ b/tests/tests/basic-truncate.sh
@@ -11,8 +11,13 @@ FILE="$T_D0/file"
 # final block as we truncated past it.
 #
 echo "== truncate writes zeroed partial end of file block"
-yes | dd of="$FILE" bs=8K count=1 status=none
+yes | dd of="$FILE" bs=8K count=1 status=none iflag=fullblock
 sync
+
+# not passing iflag=fullblock causes the file occasionally to just be
+# 4K, so just to be safe we should at least check size once
+test `stat --printf="%s\n" "$FILE"` -eq 8192 || t_fail "test file incorrect start size"
+
 truncate -s 6K "$FILE"
 truncate -s 12K "$FILE"
 echo 3 > /proc/sys/vm/drop_caches