diff --git a/src/pip/_internal/cli/req_command.py b/src/pip/_internal/cli/req_command.py
index 42888a1228b..7f41961b393 100644
--- a/src/pip/_internal/cli/req_command.py
+++ b/src/pip/_internal/cli/req_command.py
@@ -10,7 +10,7 @@
 import sys
 from functools import partial
 from optparse import Values
-from typing import TYPE_CHECKING, Any, Dict, List, Optional, Tuple, Union
+from typing import TYPE_CHECKING, Any, List, Optional, Tuple
 
 from pip._internal.cache import WheelCache
 from pip._internal.cli import cmdoptions
@@ -441,13 +441,9 @@ def get_requirements(
                     else None
                 )
                 cli_config_settings = getattr(options, "config_settings", None)
-                config_settings: Optional[Dict[str, Union[str, List[str]]]]
-                if req_config_settings and cli_config_settings:
-                    config_settings = merge_config_settings(
-                        req_config_settings, cli_config_settings
-                    )
-                else:
-                    config_settings = req_config_settings or cli_config_settings
+                config_settings = merge_config_settings(
+                    req_config_settings, cli_config_settings
+                )
                 req_to_add = install_req_from_parsed_requirement(
                     parsed_req,
                     isolated=options.isolated_mode,
diff --git a/src/pip/_internal/utils/misc.py b/src/pip/_internal/utils/misc.py
index 4d288cd8859..1f95a5624db 100644
--- a/src/pip/_internal/utils/misc.py
+++ b/src/pip/_internal/utils/misc.py
@@ -27,7 +27,6 @@
     Iterable,
     Iterator,
     List,
-    Mapping,
     Optional,
     TextIO,
     Tuple,
@@ -733,9 +732,12 @@ def prepare_metadata_for_build_editable(
 
 
 def merge_config_settings(
-    reqs_settings: Mapping[str, Union[str, List[str]]],
-    cli_settings: Mapping[str, Union[str, List[str]]],
+    reqs_settings: Optional[Dict[str, Union[str, List[str]]]],
+    cli_settings: Optional[Dict[str, Union[str, List[str]]]],
 ) -> Dict[str, Union[str, List[str]]]:
+    if not reqs_settings or not cli_settings:
+        return reqs_settings or cli_settings
+
     dd: Dict[str, Union[str, List[str]]] = {}
     for d in (reqs_settings, cli_settings):
         for k, v in d.items():
diff --git a/tests/unit/test_utils_misc.py b/tests/unit/test_utils_misc.py
index a00d5461f6d..408ee40ac38 100644
--- a/tests/unit/test_utils_misc.py
+++ b/tests/unit/test_utils_misc.py
@@ -1,25 +1,25 @@
 from typing import Dict, List, Union
 
+import pytest
+
 from pip._internal.utils.misc import merge_config_settings
 
 
-def test_merge_config_settings() -> None:
-    reqs: Dict[str, Union[str, List[str]]] = {
-        "foo": "bar",
-        "bar": "foo",
-        "foobar": ["bar"],
-        "baz": ["foo"],
-    }
-    cli: Dict[str, Union[str, List[str]]] = {
-        "foo": ["baz"],
-        "bar": "bar",
-        "foobar": ["baz"],
-        "baz": "bar",
-    }
-    expected = {
-        "foo": ["bar", "baz"],
-        "bar": ["foo", "bar"],
-        "foobar": ["bar", "baz"],
-        "baz": ["foo", "bar"],
-    }
+@pytest.mark.parametrize(
+    "reqs, cli, expected",
+    [
+        ({"foo": "bar"}, {"foo": ["baz"]}, {"foo": ["bar", "baz"]}),
+        ({"foo": "bar"}, {"foo": "baz"}, {"foo": ["bar", "baz"]}),
+        ({"foo": ["bar"]}, {"foo": ["baz"]}, {"foo": ["bar", "baz"]}),
+        ({"foo": ["bar"]}, {"foo": "baz"}, {"foo": ["bar", "baz"]}),
+        ({"foo": "bar"}, {"foo": ["baz"]}, {"foo": ["bar", "baz"]}),
+        ({"foo": "bar"}, {}, {"foo": "bar"}),
+        ({}, {"foo": ["bar"]}, {"foo": ["bar"]}),
+    ],
+)
+def test_merge_config_settings(
+    reqs: Dict[str, Union[str, List[str]]],
+    cli: Dict[str, Union[str, List[str]]],
+    expected: Dict[str, Union[str, List[str]]],
+) -> None:
     assert merge_config_settings(reqs, cli) == expected