diff --git a/youtube_dlc/__init__.py b/youtube_dlc/__init__.py
index 2072165ce6..5bf54b5563 100644
--- a/youtube_dlc/__init__.py
+++ b/youtube_dlc/__init__.py
@@ -326,7 +326,12 @@ def parse_retries(retries):
             'exec_cmd': opts.exec_cmd,
         })
 
+    _args_compat_warning = 'WARNING: %s given without specifying name. The arguments will be given to all %s\n'
+    if 'default' in opts.external_downloader_args:
+        write_string(_args_compat_warning % ('--external-downloader-args', 'external downloaders'), out=sys.stderr),
+
     if 'default-compat' in opts.postprocessor_args and 'default' not in opts.postprocessor_args:
+        write_string(_args_compat_warning % ('--post-processor-args', 'post-processors'), out=sys.stderr),
         opts.postprocessor_args.setdefault('sponskrub', [])
         opts.postprocessor_args['default'] = opts.postprocessor_args['default-compat']
 
diff --git a/youtube_dlc/options.py b/youtube_dlc/options.py
index cb8e8c06dd..7a30882f19 100644
--- a/youtube_dlc/options.py
+++ b/youtube_dlc/options.py
@@ -640,7 +640,9 @@ def _dict_from_multiple_values_options_callback(
         '--downloader-args', '--external-downloader-args',
         metavar='NAME:ARGS', dest='external_downloader_args', default={}, type='str',
         action='callback', callback=_dict_from_multiple_values_options_callback,
-        callback_kwargs={'default_key': 'default', 'process': compat_shlex_split},
+        callback_kwargs={
+            'allowed_keys': '|'.join(list_external_downloaders()), 
+            'default_key': 'default', 'process': compat_shlex_split},
         help=(
             'Give these arguments to the external downloader. '
             'Specify the downloader name and the arguments separated by a colon ":". '