diff --git a/scripts/lectures.py b/scripts/lectures.py
index b027cc4..5c2ea83 100755
--- a/scripts/lectures.py
+++ b/scripts/lectures.py
@@ -84,19 +84,19 @@ class Lectures(list):
             return self[-1].number - 1
 
     def parse_range_string_section(self, arg):
-        all_numbers = [lecture.number for lecture in self]
         if 'all' in arg:
-            return all_numbers
+            return [lecture.number for lecture in self]
 
         if '-' in arg:
             start, end = [self.parse_lecture_spec(bit) for bit in arg.split('-')]
-            return list(set(all_numbers) & set(range(start, end + 1)))
+            return list(range(start, end + 1))
 
         return [self.parse_lecture_spec(arg)]
 
     def parse_range_string(self, arg):
+        all_numbers = [lecture.number for lecture in self]
         sets = [set(self.parse_range_string_section(part)) for part in arg.split(',')]
-        return list(set.union(*sets))
+        return list(set.union(*sets) & set(all_numbers))
 
     def new_lecture(self):
         if len(self) != 0: