diff --git a/src/hb-blob.cc b/src/hb-blob.cc
index 51f22ce..4b036e8 100644
--- a/src/hb-blob.cc
+++ b/src/hb-blob.cc
@@ -57,8 +57,6 @@
 {
   HB_OBJECT_HEADER_STATIC,
 
-  true, /* immutable */
-
   nullptr, /* data */
   0, /* length */
   HB_MEMORY_MODE_READONLY, /* mode */
@@ -299,12 +297,10 @@
 void
 hb_blob_make_immutable (hb_blob_t *blob)
 {
-  if (hb_object_is_inert (blob))
-    return;
-  if (blob->immutable)
+  if (hb_object_is_immutable (blob))
     return;
 
-  blob->immutable = true;
+  hb_object_make_immutable (blob);
 }
 
 /**
@@ -320,7 +316,7 @@
 hb_bool_t
 hb_blob_is_immutable (hb_blob_t *blob)
 {
-  return blob->immutable;
+  return hb_object_is_immutable (blob);
 }
 
 
@@ -454,7 +450,7 @@
 bool
 hb_blob_t::try_make_writable (void)
 {
-  if (this->immutable)
+  if (hb_object_is_immutable (this))
     return false;
 
   if (this->mode == HB_MEMORY_MODE_WRITABLE)