diff --git a/src/hb-ot-hdmx-table.hh b/src/hb-ot-hdmx-table.hh
index c523c420a..87dd6d018 100644
--- a/src/hb-ot-hdmx-table.hh
+++ b/src/hb-ot-hdmx-table.hh
@@ -88,8 +88,13 @@ struct DeviceRecord
   {
     TRACE_SERIALIZE (this);
 
-    if (unlikely (!c->allocate_size<DeviceRecord> (get_size (subset_view.len()))))
+    unsigned int size = get_size (subset_view.len());
+    if (unlikely (!c->allocate_size<DeviceRecord> (size)))
+    {
+      DEBUG_MSG (SUBSET, nullptr, "Couldn't allocate enough space for DeviceRecord: %d.",
+                 size);
       return_trace (false);
+    }
 
     this->pixel_size.set (subset_view.source_device_record->pixel_size);
     this->max_width.set (subset_view.source_device_record->max_width);
@@ -160,14 +165,14 @@ struct hdmx
     return_trace (true);
   }
 
-  static inline size_t get_subsetted_size (hb_subset_plan_t *plan)
+  static inline size_t get_subsetted_size (const hdmx *source_hdmx, hb_subset_plan_t *plan)
   {
-    return min_size + DeviceRecord::get_size (plan->glyphs.len);
+    return min_size + source_hdmx->num_records * DeviceRecord::get_size (plan->glyphs.len);
   }
 
   inline bool subset (hb_subset_plan_t *plan) const
   {
-    size_t dest_size = get_subsetted_size (plan);
+    size_t dest_size = get_subsetted_size (this, plan);
     hdmx *dest = (hdmx *) malloc (dest_size);
     if (unlikely (!dest))
     {
@@ -180,6 +185,7 @@ struct hdmx
     if (!hdmx_prime || !hdmx_prime->serialize (&c, this, plan))
     {
       free (dest);
+      DEBUG_MSG(SUBSET, nullptr, "Failed to serialize write new hdmx.");
       return false;
     }
     c.end_serialize ();
diff --git a/test/api/fonts/Roboto-Regular.multihdmx.a.ttf b/test/api/fonts/Roboto-Regular.multihdmx.a.ttf
new file mode 100644
index 000000000..dd82178fd
Binary files /dev/null and b/test/api/fonts/Roboto-Regular.multihdmx.a.ttf differ
diff --git a/test/api/fonts/Roboto-Regular.multihdmx.abc.ttf b/test/api/fonts/Roboto-Regular.multihdmx.abc.ttf
new file mode 100644
index 000000000..03dd8b6b7
Binary files /dev/null and b/test/api/fonts/Roboto-Regular.multihdmx.abc.ttf differ
diff --git a/test/api/test-subset-hdmx.c b/test/api/test-subset-hdmx.c
index 22fcb4118..8a1e821a3 100644
--- a/test/api/test-subset-hdmx.c
+++ b/test/api/test-subset-hdmx.c
@@ -50,6 +50,25 @@ test_subset_hdmx_simple_subset (void)
   hb_face_destroy (face_ac);
 }
 
+static void
+test_subset_hdmx_multiple_device_records (void)
+{
+  hb_face_t *face_abc = hb_subset_test_open_font ("fonts/Roboto-Regular.multihdmx.abc.ttf");
+  hb_face_t *face_a = hb_subset_test_open_font ("fonts/Roboto-Regular.multihdmx.a.ttf");
+
+  hb_set_t *codepoints = hb_set_create ();
+  hb_face_t *face_abc_subset;
+  hb_set_add (codepoints, 'a');
+  face_abc_subset = hb_subset_test_create_subset (face_abc, hb_subset_test_create_input (codepoints));
+  hb_set_destroy (codepoints);
+
+  hb_subset_test_check (face_a, face_abc_subset, HB_TAG ('h','d','m','x'));
+
+  hb_face_destroy (face_abc_subset);
+  hb_face_destroy (face_abc);
+  hb_face_destroy (face_a);
+}
+
 static void
 test_subset_hdmx_invalid (void)
 {
@@ -115,6 +134,7 @@ main (int argc, char **argv)
   hb_test_init (&argc, &argv);
 
   hb_test_add (test_subset_hdmx_simple_subset);
+  hb_test_add (test_subset_hdmx_multiple_device_records);
   hb_test_add (test_subset_hdmx_invalid);
   hb_test_add (test_subset_hdmx_fails_sanitize);
   hb_test_add (test_subset_hdmx_noop);