diff --git a/tests/scripts/check_test_cases.py b/tests/scripts/check_test_cases.py
index 6069a66..213708b 100755
--- a/tests/scripts/check_test_cases.py
+++ b/tests/scripts/check_test_cases.py
@@ -120,6 +120,8 @@
         # Assume compat.sh is responsible for printing identical format of
         # test case description between --list-test-case and its OUTCOME.CSV
         description = compat_output.strip().split(b'\n')
+        # idx indicates the number of test case since there is no line number
+        # in `compat.sh` for each test case.
         for idx, descrip in enumerate(description):
             self.process_test_case(descriptions, file_name, idx, descrip)