Validate input

src/dials/command_line/export_bitmaps.py

@@ -173,6 +173,10 @@ def imageset_as_bitmaps(imageset, params):
     if params.imageset_index:
         selected_images = []
         for idx in params.imageset_index:
+            if idx > len(images):
+                sys.exit(
+                    f"Bad value for imageset_index: {idx}. The imageset has length {len(images)}; allowable values for imageset_index are the range 1 to {len(images)} inclusive."
+                )
             selected_images.append(images[idx - 1])
         images = selected_images
 

src/dials/util/export_bitmaps.py

@@ -87,7 +87,12 @@ def imageset_as_flex_image(
         start = 1
 
     # If the user specified an image range index, only export those
+    n_images = len(imageset)
     for i_image in images:
+        if (i_image < start) or (i_image >= start + n_images):
+            raise ValueError(
+                f"Image {i_image} outside of scan range {start},{start+n_images-1}"
+            )
         image = imageset.get_raw_data(i_image - start)
 
         mask = imageset.get_mask(i_image - start)