diff --git a/src/hb-ot-hmtx-table.hh b/src/hb-ot-hmtx-table.hh
index 7c2223c64..6035a33fb 100644
--- a/src/hb-ot-hmtx-table.hh
+++ b/src/hb-ot-hmtx-table.hh
@@ -272,6 +272,7 @@ struct hmtxvmtx
     }
 
     protected:
+    // 0 <= num_long_metrics <= num_bearings
     unsigned int num_long_metrics;
     unsigned int num_bearings;
     unsigned int default_advance;