diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 429e533e..a88cb63b 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -23,9 +23,11 @@ Added
 Fixed
 ^^^^^
 - Parameter resolving falling back to assumptions resolver for optional
-  ``Union`` types.
+  ``Union`` types (`#498 <https://github.com/omni-us/jsonargparse/pull/498>`__).
 - Nested parameters failing to parse from command line when value includes
-  space.
+  space (`#499 <https://github.com/omni-us/jsonargparse/pull/499>`__).
+- ``format_usage()`` not working (`#501
+  <https://github.com/omni-us/jsonargparse/issues/501>`__).
 
 
 v4.28.0 (2024-04-17)
diff --git a/jsonargparse/_common.py b/jsonargparse/_common.py
index 9a5b2b05..36d558e2 100644
--- a/jsonargparse/_common.py
+++ b/jsonargparse/_common.py
@@ -51,7 +51,7 @@ def __call__(self, class_type: Type[ClassType], *args, **kwargs) -> ClassType:
 InstantiatorsDictType = Dict[Tuple[type, bool], InstantiatorCallable]
 
 
-parent_parser: ContextVar["ArgumentParser"] = ContextVar("parent_parser")
+parent_parser: ContextVar[Optional["ArgumentParser"]] = ContextVar("parent_parser", default=None)
 parser_capture: ContextVar[bool] = ContextVar("parser_capture", default=False)
 defaults_cache: ContextVar[Optional[Namespace]] = ContextVar("defaults_cache", default=None)
 lenient_check: ContextVar[Union[bool, str]] = ContextVar("lenient_check", default=False)
diff --git a/jsonargparse/_formatters.py b/jsonargparse/_formatters.py
index 408e0749..6b86aa5e 100644
--- a/jsonargparse/_formatters.py
+++ b/jsonargparse/_formatters.py
@@ -88,17 +88,19 @@ def _get_help_string(self, action: Action) -> str:
     def _format_usage(self, *args, **kwargs) -> str:
         usage = super()._format_usage(*args, **kwargs)
         parser = parent_parser.get()
-        for key in parser.required_args:
-            try:
-                default = parser.get_default(key)
-            except KeyError:
-                default = None
-            if default is None and f"[--{key} " in usage:
-                usage = re.sub(f"\\[(--{key} [^\\]]+)]", r"\1", usage, count=1)
+        if parser:
+            for key in parser.required_args:
+                try:
+                    default = parser.get_default(key)
+                except KeyError:
+                    default = None
+                if default is None and f"[--{key} " in usage:
+                    usage = re.sub(f"\\[(--{key} [^\\]]+)]", r"\1", usage, count=1)
         return usage
 
     def _format_action_invocation(self, action: Action) -> str:
         parser = parent_parser.get()
+        assert parser is not None
         if isinstance(action, _ActionSubCommands):
             value = "Available subcommands:"
             if parser.default_env:
@@ -263,6 +265,7 @@ def get_env_var(
         parser = parent_parser.get()
     else:
         parser = parser_or_formatter
+    assert parser is not None
     env_var = ""
     if isinstance(parser.env_prefix, str):
         env_var = parser.env_prefix.replace("-", "_") + "_"
diff --git a/jsonargparse/_loaders_dumpers.py b/jsonargparse/_loaders_dumpers.py
index c22e5509..8d7ff057 100644
--- a/jsonargparse/_loaders_dumpers.py
+++ b/jsonargparse/_loaders_dumpers.py
@@ -96,7 +96,9 @@ def jsonnet_load(stream, path="", ext_vars=None):
 def get_load_value_mode() -> str:
     mode = load_value_mode.get()
     if mode is None:
-        mode = parent_parser.get().parser_mode
+        parser = parent_parser.get()
+        assert parser is not None
+        mode = parser.parser_mode
     return mode
 
 
diff --git a/jsonargparse_tests/test_formatters.py b/jsonargparse_tests/test_formatters.py
index 26960d82..c6f175e6 100644
--- a/jsonargparse_tests/test_formatters.py
+++ b/jsonargparse_tests/test_formatters.py
@@ -1,7 +1,9 @@
 from __future__ import annotations
 
+import os
 from pathlib import Path
 from typing import Tuple
+from unittest.mock import patch
 
 import pytest
 
@@ -129,3 +131,9 @@ def test_help_default_config_files_with_required(tmp_path, parser):
     help_str = get_parser_help(parser)
     assert "req description" in help_str
     assert "from yaml" in help_str
+
+
+def test_format_usage(parser):
+    parser.add_argument("--v1")
+    with patch.dict(os.environ, {"COLUMNS": "200"}):
+        assert parser.format_usage() == "usage: app [-h] [--v1 V1]\n"