diff --git a/src/hb-subset-plan.cc b/src/hb-subset-plan.cc
index 0c11765c5..d70215b00 100644
--- a/src/hb-subset-plan.cc
+++ b/src/hb-subset-plan.cc
@@ -40,7 +40,7 @@ _hb_codepoint_t_cmp (const void *pa, const void *pb)
 }
 
 hb_bool_t
-hb_subset_plan_new_gid_for_codepoint (hb_subset_plan_t *plan,
+hb_subset_plan_new_gid_for_codepoint (const hb_subset_plan_t *plan,
                                       hb_codepoint_t codepoint,
                                       hb_codepoint_t *new_gid)
 {
@@ -58,7 +58,7 @@ hb_subset_plan_new_gid_for_codepoint (hb_subset_plan_t *plan,
 }
 
 hb_bool_t
-hb_subset_plan_new_gid_for_old_id (hb_subset_plan_t *plan,
+hb_subset_plan_new_gid_for_old_id (const hb_subset_plan_t *plan,
                                    hb_codepoint_t old_gid,
                                    hb_codepoint_t *new_gid)
 {
diff --git a/src/hb-subset-plan.hh b/src/hb-subset-plan.hh
index 3b9db03f2..1c8697b66 100644
--- a/src/hb-subset-plan.hh
+++ b/src/hb-subset-plan.hh
@@ -65,19 +65,19 @@ hb_subset_plan_create (hb_face_t           *face,
                        hb_subset_input_t   *input);
 
 HB_INTERNAL hb_bool_t
-hb_subset_plan_new_gid_for_old_id(hb_subset_plan_t *plan,
-                                  hb_codepoint_t old_gid,
-                                  hb_codepoint_t *new_gid /* OUT */);
+hb_subset_plan_new_gid_for_old_id (const hb_subset_plan_t *plan,
+                                   hb_codepoint_t old_gid,
+                                   hb_codepoint_t *new_gid /* OUT */);
 
 HB_INTERNAL hb_bool_t
-hb_subset_plan_new_gid_for_codepoint(hb_subset_plan_t *plan,
-                                     hb_codepoint_t codepont,
-                                     hb_codepoint_t *new_gid /* OUT */);
+hb_subset_plan_new_gid_for_codepoint (const hb_subset_plan_t *plan,
+                                      hb_codepoint_t codepont,
+                                      hb_codepoint_t *new_gid /* OUT */);
 
 HB_INTERNAL hb_bool_t
-hb_subset_plan_add_table(hb_subset_plan_t *plan,
-                         hb_tag_t tag,
-                         hb_blob_t *contents);
+hb_subset_plan_add_table (hb_subset_plan_t *plan,
+                          hb_tag_t tag,
+                          hb_blob_t *contents);
 
 HB_INTERNAL void
 hb_subset_plan_destroy (hb_subset_plan_t *plan);