diff --git a/tests/valid/datetime/datetime.toml b/tests/valid/datetime/datetime.toml
index 653d521..831d261 100644
--- a/tests/valid/datetime/datetime.toml
+++ b/tests/valid/datetime/datetime.toml
@@ -1,2 +1,4 @@
 space = 1987-07-05 17:45:00Z
+
+# ABNF is case-insensitive, both "Z" and "z" must be supported.
 lower = 1987-07-05t17:45:00z