diff --git a/src/hb-aat-layout-common.hh b/src/hb-aat-layout-common.hh
index 6a4952f0b..e0cfb078e 100644
--- a/src/hb-aat-layout-common.hh
+++ b/src/hb-aat-layout-common.hh
@@ -39,6 +39,7 @@ namespace AAT {
 
 using namespace OT;
 
+#define HB_AAT_BUFFER_DIGEST_THRESHOLD 32
 
 struct ankr;
 
diff --git a/src/hb-aat-layout-kerx-table.hh b/src/hb-aat-layout-kerx-table.hh
index 065c06b36..940ba0c4a 100644
--- a/src/hb-aat-layout-kerx-table.hh
+++ b/src/hb-aat-layout-kerx-table.hh
@@ -977,7 +977,7 @@ struct KerxTable
   {
     c->buffer->unsafe_to_concat ();
 
-    if (c->buffer->len < 16)
+    if (c->buffer->len < HB_AAT_BUFFER_DIGEST_THRESHOLD)
       c->buffer_digest = c->buffer->digest ();
     else
       c->buffer_digest = hb_set_digest_t::full ();
diff --git a/src/hb-aat-layout-morx-table.hh b/src/hb-aat-layout-morx-table.hh
index f3d42f4e6..4a0f6da16 100644
--- a/src/hb-aat-layout-morx-table.hh
+++ b/src/hb-aat-layout-morx-table.hh
@@ -1374,7 +1374,7 @@ struct mortmorx
 
     c->buffer->unsafe_to_concat ();
 
-    if (c->buffer->len < 16)
+    if (c->buffer->len < HB_AAT_BUFFER_DIGEST_THRESHOLD)
       c->buffer_digest = c->buffer->digest ();
     else
       c->buffer_digest = hb_set_digest_t::full ();