diff --git a/Lib/fontTools/varLib/__init__.py b/Lib/fontTools/varLib/__init__.py
index 0708ff2b1..0677a0995 100644
--- a/Lib/fontTools/varLib/__init__.py
+++ b/Lib/fontTools/varLib/__init__.py
@@ -205,7 +205,7 @@ def _SetCoordinates(font, glyphName, coord):
 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[i+1:j], coords[i], deltas[i], coords[j], deltas[j])
+	interp = list(_iup_segment(coords[i+1:j], coords[i], deltas[i], coords[j], deltas[j]))
 	deltas = deltas[i+1:j]
 
 	assert len(deltas) == len(interp)