diff --git a/src/hb-open-type.hh b/src/hb-open-type.hh
index 4ff69c7a3..16bf005c8 100644
--- a/src/hb-open-type.hh
+++ b/src/hb-open-type.hh
@@ -213,6 +213,11 @@ typedef Index NameID;
 struct VarIdx : HBUINT32 {
   static constexpr unsigned NO_VARIATION = 0xFFFFFFFFu;
   static_assert (NO_VARIATION == HB_OT_LAYOUT_NO_VARIATIONS_INDEX, "");
+  static uint32_t add (uint32_t i, unsigned short v)
+  {
+    if (i == NO_VARIATION) return i;
+    return i + v;
+  }
   VarIdx& operator = (uint32_t i) { HBUINT32::operator= (i); return *this; }
 };
 DECLARE_NULL_NAMESPACE_BYTES (OT, VarIdx);
diff --git a/src/hb-ot-color-colr-table.hh b/src/hb-ot-color-colr-table.hh
index 43f17c01a..545448534 100644
--- a/src/hb-ot-color-colr-table.hh
+++ b/src/hb-ot-color-colr-table.hh
@@ -940,13 +940,13 @@ struct ClipBox
       int ymin = u.format1.yMin;
       int xmax = u.format1.xMax;
       int ymax = u.format1.yMax;
-      if (u.format == 2 && instancer && u.format2.varIdxBase != VarIdx::NO_VARIATION)
+      if (u.format == 2 && instancer)
       {
         uint32_t varIdx = u.format2.varIdxBase;
-        xmin += _hb_roundf (instancer (varIdx+0));
-        ymin += _hb_roundf (instancer (varIdx+1));
-        xmax += _hb_roundf (instancer (varIdx+2));
-        ymax += _hb_roundf (instancer (varIdx+3));
+        xmin += _hb_roundf (instancer (VarIdx::add (varIdx, 0)));
+        ymin += _hb_roundf (instancer (VarIdx::add (varIdx, 1)));
+        xmax += _hb_roundf (instancer (VarIdx::add (varIdx, 2)));
+        ymax += _hb_roundf (instancer (VarIdx::add (varIdx, 3)));
       }
       extents->x_bearing = xmin;
       extents->y_bearing = ymax;