diff --git a/Lib/fontTools/varLib/instancer/__init__.py b/Lib/fontTools/varLib/instancer/__init__.py
index d1cde0df7..df55f0414 100644
--- a/Lib/fontTools/varLib/instancer/__init__.py
+++ b/Lib/fontTools/varLib/instancer/__init__.py
@@ -614,7 +614,7 @@ def _instantiateGvarGlyph(
     if optimize:
         isComposite = glyf[glyphname].isComposite()
         for var in tupleVarStore:
-            var.optimize(coordinates, endPts, isComposite)
+            var.optimize(coordinates, endPts, isComposite=isComposite)
 
 
 def instantiateGvarGlyph(varfont, glyphname, axisLimits, optimize=True):