diff --git a/utils/raspberrypi/ctt/ctt.py b/utils/raspberrypi/ctt/ctt.py
index 96f1b5e6..186afda5 100755
--- a/utils/raspberrypi/ctt/ctt.py
+++ b/utils/raspberrypi/ctt/ctt.py
@@ -198,9 +198,12 @@ class Camera:
         """
         Write output to json
         """
-        self.json['rpi.cac']['cac'] = cacs
-        self.log += '\nCAC calibration written to json file'
-        print('Finished CAC calibration')
+        if cacs:
+            self.json['rpi.cac']['cac'] = cacs
+            self.log += '\nCAC calibration written to json file'
+            print('Finished CAC calibration')
+        else:
+            self.log += "\nCAC calibration failed"
 
 
     """
diff --git a/utils/raspberrypi/ctt/ctt_cac.py b/utils/raspberrypi/ctt/ctt_cac.py
index 5a4c5101..a1183989 100644
--- a/utils/raspberrypi/ctt/ctt_cac.py
+++ b/utils/raspberrypi/ctt/ctt_cac.py
@@ -108,12 +108,29 @@ def shifts_to_yaml(red_shift, blue_shift, image_dimensions, output_grid_size=9):
         ybsgrid[xgridloc][ygridloc].append(blue_shift[3])
 
     # Now calculate the average pixel shift for each square in the grid
+    grid_incomplete = False
     for x in range(output_grid_size - 1):
         for y in range(output_grid_size - 1):
-            xrgrid[x, y] = np.mean(xrsgrid[x][y])
-            yrgrid[x, y] = np.mean(yrsgrid[x][y])
-            xbgrid[x, y] = np.mean(xbsgrid[x][y])
-            ybgrid[x, y] = np.mean(ybsgrid[x][y])
+            if xrsgrid[x][y]:
+                xrgrid[x, y] = np.mean(xrsgrid[x][y])
+            else:
+                grid_incomplete = True
+            if yrsgrid[x][y]:
+                yrgrid[x, y] = np.mean(yrsgrid[x][y])
+            else:
+                grid_incomplete = True
+            if xbsgrid[x][y]:
+                xbgrid[x, y] = np.mean(xbsgrid[x][y])
+            else:
+                grid_incomplete = True
+            if ybsgrid[x][y]:
+                ybgrid[x, y] = np.mean(ybsgrid[x][y])
+            else:
+                grid_incomplete = True
+
+    if grid_incomplete:
+        raise RuntimeError("\nERROR: CAC measurements do not span the image!"
+                           "\nConsider using improved CAC images, or remove them entirely.\n")
 
     # Next, we start to interpolate the central points of the grid that gets passed to the tuning file
     input_grids = np.array([xrgrid, yrgrid, xbgrid, ybgrid])
@@ -219,7 +236,12 @@ def cac(Cam):
     # tuning file
     print("\nCreating output grid")
     Cam.log += '\nCreating output grid'
-    rx, ry, bx, by = shifts_to_yaml(red_shift, blue_shift, image_size)
+    try:
+        rx, ry, bx, by = shifts_to_yaml(red_shift, blue_shift, image_size)
+    except RuntimeError as e:
+        print(str(e))
+        Cam.log += "\nCAC correction failed! CAC will not be enabled."
+        return {}
 
     print("CAC correction complete!")
     Cam.log += '\nCAC correction complete!'