diff --git a/src/hb-ot-var-common.hh b/src/hb-ot-var-common.hh
index 5c82ae159..de4ec322a 100644
--- a/src/hb-ot-var-common.hh
+++ b/src/hb-ot-var-common.hh
@@ -270,6 +270,7 @@ struct TupleVariationHeader
 
       if (shared_tuple_active_idx)
       {
+        assert (index < shared_tuple_active_idx->length);
 	int v = (*shared_tuple_active_idx).arrayZ[index];
 	if (v != -1)
 	{