diff --git a/Lib/fontTools/varLib/instancer/__init__.py b/Lib/fontTools/varLib/instancer/__init__.py
index 0c5dd40ce..526060875 100644
--- a/Lib/fontTools/varLib/instancer/__init__.py
+++ b/Lib/fontTools/varLib/instancer/__init__.py
@@ -1880,7 +1880,7 @@ def main(args=None):
     )
 
     isFullInstance = {
-        axisTag for axisTag, limit in axisLimits.items() if limit[0] == limit[2]
+        axisTag for axisTag, limit in axisLimits.items() if not isinstance(limit, tuple)
     }.issuperset(axis.axisTag for axis in varfont["fvar"].axes)
 
     instantiateVariableFont(