diff --git a/src/hb-subset-input.cc b/src/hb-subset-input.cc
index aa1634309..70754411f 100644
--- a/src/hb-subset-input.cc
+++ b/src/hb-subset-input.cc
@@ -402,6 +402,8 @@ hb_subset_input_get_user_data (const hb_subset_input_t *input,
  * @axis_tag: Tag of the axis to be pinned
  *
  * Pin an axis to its default location in the given subset input object.
+ * Currently only works for fonts with 'glyf' tables. CFF and CFF2 is not
+ * yet supported.
  *
  * Return value: `true` if success, `false` otherwise
  *
@@ -426,6 +428,8 @@ hb_subset_input_pin_axis_to_default (hb_subset_input_t  *input,
  * @axis_value: Location on the axis to be pinned at
  *
  * Pin an axis to a fixed location in the given subset input object.
+ * Currently only works for fonts with 'glyf' tables. CFF and CFF2 is not
+ * yet supported.
  *
  * Return value: `true` if success, `false` otherwise
  *