diff --git a/chatmastermind/commands/question.py b/chatmastermind/commands/question.py
index bc4a8c4..ba522ab 100644
--- a/chatmastermind/commands/question.py
+++ b/chatmastermind/commands/question.py
@@ -151,13 +151,34 @@ def repeat_messages(messages: list[Message], chat: ChatDB, args: argparse.Namesp
             make_request(ai, chat, message, args)
 
 
+def modify_tag_args(args: argparse.Namespace) -> None:
+    """
+    Changes the semantics of the tags for this command:
+    * not tags specified -> no tags are selected
+    * empty tags specified -> all tags are selected
+    """
+    if args.or_tags is None:
+        args.or_tags = set()
+    elif len(args.or_tags) == 0:
+        args.or_tags = None
+    if args.and_tags is None:
+        args.and_tags = set()
+    elif len(args.and_tags) == 0:
+        args.and_tags = None
+    if args.exclude_tags is None:
+        args.exclude_tags = set()
+    elif len(args.exclude_tags) == 0:
+        args.exclude_tags = None
+
+
 def question_cmd(args: argparse.Namespace, config: Config) -> None:
     """
     Handler for the 'question' command.
     """
-    mfilter = MessageFilter(tags_or=args.or_tags if args.or_tags is not None else set(),
-                            tags_and=args.and_tags if args.and_tags is not None else set(),
-                            tags_not=args.exclude_tags if args.exclude_tags is not None else set())
+    modify_tag_args(args)
+    mfilter = MessageFilter(tags_or=args.or_tags,
+                            tags_and=args.and_tags,
+                            tags_not=args.exclude_tags)
     chat = ChatDB.from_dir(cache_path=Path(config.cache),
                            db_path=Path(config.db),
                            mfilter=mfilter)
diff --git a/chatmastermind/main.py b/chatmastermind/main.py
index 62c4539..ac4f7cc 100755
--- a/chatmastermind/main.py
+++ b/chatmastermind/main.py
@@ -34,13 +34,13 @@ def create_parser() -> argparse.ArgumentParser:
 
     # a parent parser for all commands that support tag selection
     tag_parser = argparse.ArgumentParser(add_help=False)
-    tag_arg = tag_parser.add_argument('-t', '--or-tags', nargs='+',
+    tag_arg = tag_parser.add_argument('-t', '--or-tags', nargs='*',
                                       help='List of tags (one must match)', metavar='OTAGS')
     tag_arg.completer = tags_completer  # type: ignore
-    atag_arg = tag_parser.add_argument('-k', '--and-tags', nargs='+',
+    atag_arg = tag_parser.add_argument('-k', '--and-tags', nargs='*',
                                        help='List of tags (all must match)', metavar='ATAGS')
     atag_arg.completer = tags_completer  # type: ignore
-    etag_arg = tag_parser.add_argument('-x', '--exclude-tags', nargs='+',
+    etag_arg = tag_parser.add_argument('-x', '--exclude-tags', nargs='*',
                                        help='List of tags to exclude', metavar='XTAGS')
     etag_arg.completer = tags_completer  # type: ignore
     otag_arg = tag_parser.add_argument('-o', '--output-tags', nargs='+',