diff --git a/Lib/fontTools/varLib/models.py b/Lib/fontTools/varLib/models.py
index 46eeb0d53..2bdc0e4c6 100644
--- a/Lib/fontTools/varLib/models.py
+++ b/Lib/fontTools/varLib/models.py
@@ -274,6 +274,9 @@ class VariationModel(object):
 					elif locV < val:
 						newUpper = val
 						percentage = (val - locV) / (upper - locV)
+					else: # val == locV
+						# Can't split box in this direction.
+						continue
 					if percentage > bestPercentage:
 						bestPercentage = percentage
 						bestAxis = axis