diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index e9507a4a..5c62d1b8 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -30,6 +30,9 @@ Fixed
 - While parsing, internally preserve ``class_path`` for subclass disabled types
   to correctly support defaults from union of dataclasses (`#921
   <https://github.com/omni-us/jsonargparse/pull/921>`__).
+- Misleading error message for ``parse_optionals_as_positionals=True`` and
+  unrecognized non-positional arguments (`#922
+  <https://github.com/omni-us/jsonargparse/pull/922>`__).
 
 Changed
 ^^^^^^^
diff --git a/jsonargparse/_core.py b/jsonargparse/_core.py
index 81601287..f4c5335d 100644
--- a/jsonargparse/_core.py
+++ b/jsonargparse/_core.py
@@ -314,12 +314,19 @@ def _positional_optionals(self, cfg, unk):
 
         for action in get_optionals_as_positionals_actions(self, include_positionals=True):
             if action.option_strings == []:
-                if cfg.get(action.dest) is None:
+                if cfg.get(action.dest) is get_parsing_setting("unset_sentinel"):
                     self._logger.debug(f"Positional argument {action.dest} missing, aborting _positional_optionals")
                     break
                 continue
 
-            cfg[action.dest] = self._check_value_key(action, unk.pop(0), action.dest, cfg)
+            value = unk.pop(0)
+            try:
+                cfg[action.dest] = self._check_value_key(action, value, action.dest, cfg)
+            except (TypeError, ValueError) as ex:
+                if isinstance(value, str) and value.startswith("--"):
+                    raise argument_error(f"unrecognized arguments: {' '.join([value] + unk)}") from ex
+                raise
+
             if len(unk) == 0:
                 break
 
diff --git a/jsonargparse_tests/test_parsing_settings.py b/jsonargparse_tests/test_parsing_settings.py
index 8dd581bf..ce890d49 100644
--- a/jsonargparse_tests/test_parsing_settings.py
+++ b/jsonargparse_tests/test_parsing_settings.py
@@ -115,6 +115,12 @@ def test_parse_optionals_as_positionals_simple(parser, logger, subtests):
                 parser.parse_args(["--unk=x"])
         assert "Positional argument p1 missing, aborting _positional_optionals" in logs.getvalue()
 
+    with subtests.test("mixed unrecognized"):
+        with capture_logs(logger) as logs:
+            with pytest.raises(ArgumentError, match="unrecognized arguments: --unexpected arg xyz"):
+                parser.parse_args(["p1", "3", "--unexpected", "arg", "xyz"])
+        assert "unrecognized arguments: --unexpected arg xyz" in logs.getvalue()
+
 
 def test_parse_optionals_as_positionals_subcommands(parser, subparser, subtests):
     set_parsing_settings(parse_optionals_as_positionals=True)
@@ -153,6 +159,10 @@ def test_parse_optionals_as_positionals_subcommands(parser, subparser, subtests)
             parser.parse_args(["subcmd", "p1", "o2", "5"])
         assert re.match('Parser key "o1".*Given value: o2', ex.value.message, re.DOTALL)
 
+    with subtests.test("mixed unrecognized"):
+        with pytest.raises(ArgumentError, match="unrecognized arguments: --unexpected=arg"):
+            parser.parse_args(["subcmd", "p1", "3", "--unexpected=arg"])
+
 
 def test_optionals_as_positionals_usage_wrap(parser):
     set_parsing_settings(parse_optionals_as_positionals=True)