diff --git a/scripts/sub-digest/subdigest.py b/scripts/sub-digest/subdigest.py
index 3f94ef8..cfc7f8c 100644
--- a/scripts/sub-digest/subdigest.py
+++ b/scripts/sub-digest/subdigest.py
@@ -117,7 +117,8 @@ def _find_line_expr(self, expr):
 
     def _get_selection(self):
         if self.selection is None:
-            return self._get_section()
+            # return copy of section contents as list
+            return list(self._get_section())
 
         return [line for i, line in enumerate(self._get_section())
                 if i in self.selection]
@@ -130,15 +131,17 @@ def _get_nonselection(self):
                 if i not in self.selection]
 
     def _process_selection(self, f):
+        lines = self._get_selection()
+
         if self.selection is None:
-            f(self._get_section())
+            indices = range(len(lines))
         else:
             indices = sorted(self.selection)
-            lines = self._get_selection()
-            f(lines)
-            section = self._get_section()
-            for i, line in zip(indices, lines):
-                section[i] = line
+
+        f(lines)
+        section = self._get_section()
+        for i, line in zip(indices, lines):
+            section[i] = line
 
     @filter
     def use_styles(self) -> Subtitles: