diff --git a/src/hb-ot-hmtx-table.hh b/src/hb-ot-hmtx-table.hh
index 16eb1eb91..53a3e2f83 100644
--- a/src/hb-ot-hmtx-table.hh
+++ b/src/hb-ot-hmtx-table.hh
@@ -92,7 +92,7 @@ struct hmtxvmtx
 
     unsigned int length;
     H *table = (H *) hb_blob_get_data (dest_blob, &length);
-    table->numberOfLongMetrics = num_hmetrics;
+    c->serializer->check_assign (table->numberOfLongMetrics, num_hmetrics, HB_SERIALIZE_ERROR_INT_OVERFLOW);
 
 #ifndef HB_NO_VAR
     if (c->plan->normalized_coords)