diff --git a/ObservationSim/Instrument/Chip/Chip.py b/ObservationSim/Instrument/Chip/Chip.py
index 2ddfa1b48fab48f5fd54a06ea54f0887508c655d..f4ce360b7f009a8766714996a943c413f3a36c49 100755
--- a/ObservationSim/Instrument/Chip/Chip.py
+++ b/ObservationSim/Instrument/Chip/Chip.py
@@ -656,7 +656,7 @@ class Chip(FocalPlane):
                     FlatCombImg = newimg
 
                     ### 1*16 -> 2*8 img-layout
-                    img = chip_utils.formatRevert(GSImage=FlatCombImg)
+                    FlatCombImg = chip_utils.formatRevert(GSImage=FlatCombImg)
                     self.nsecy = 2
                     self.nsecx = 8