diff --git a/Lib/fontTools/varLib/__init__.py b/Lib/fontTools/varLib/__init__.py
index 39daed2ae..f0ec642c7 100644
--- a/Lib/fontTools/varLib/__init__.py
+++ b/Lib/fontTools/varLib/__init__.py
@@ -202,11 +202,11 @@ def _SetCoordinates(font, glyphName, coord):
 	font["hmtx"].metrics[glyphName] = horizontalAdvanceWidth, leftSideBearing
 
 
-def _all_interpolatable_in_between(deltas, coords, j, i, tolerance):
-	assert i - j >= 2
+def _all_interpolatable_in_between(deltas, coords, i, j, tolerance):
+	assert j - i >= 2
 	from fontTools.varLib.mutator import _iup_segment
-	interp = _iup_segment(coords[j+1:i], coords[j], deltas[j], coords[i], deltas[i])
-	deltas = deltas[j+1:i]
+	interp = _iup_segment(coords[i+1:j], coords[i], deltas[i], coords[j], deltas[j])
+	deltas = deltas[i+1:j]
 
 	assert len(deltas) == len(interp)