diff --git a/script/source_format/format_python.sh b/script/source_format/format_python.sh
index 294515ef3..d136b935c 100755
--- a/script/source_format/format_python.sh
+++ b/script/source_format/format_python.sh
@@ -37,9 +37,9 @@ do
 done
 
 for SRC_DIR in "${DIRS[@]}"; do
-    isort --profile black "${CHECK}" "${SRC_DIR}"
+    isort --profile black ${CHECK:+"$CHECK"} "${SRC_DIR}"
     ((FAILURES+=$?))
-    black -l 100 "${CHECK}" "${SRC_DIR}"
+    black -l 100 ${CHECK:+"$CHECK"} "${SRC_DIR}"
     ((FAILURES+=$?))
 done