diff --git a/Lib/fontTools/varLib/builder.py b/Lib/fontTools/varLib/builder.py
index a2db14dbd..94cc5bf06 100644
--- a/Lib/fontTools/varLib/builder.py
+++ b/Lib/fontTools/varLib/builder.py
@@ -145,7 +145,7 @@ def buildDeltaSetIndexMap(varIdxes):
         return None
     self = ot.DeltaSetIndexMap()
     self.mapping = mapping
-    self.Format = 1 if len(varIdxes) > 0xFFFF else 0
+    self.Format = 1 if len(mapping) > 0xFFFF else 0
     return self