diff --git a/inboard/gunicorn_conf.py b/inboard/gunicorn_conf.py
index e1bf8b5..727e09d 100644
--- a/inboard/gunicorn_conf.py
+++ b/inboard/gunicorn_conf.py
@@ -3,13 +3,13 @@
 
 from inboard.start import configure_logging
 
-workers_per_core_str = os.getenv("WORKERS_PER_CORE", "1")
+# Gunicorn setup
 max_workers_str = os.getenv("MAX_WORKERS")
+web_concurrency_str = os.getenv("WEB_CONCURRENCY", None)
+workers_per_core_str = os.getenv("WORKERS_PER_CORE", "1")
 use_max_workers = None
-if max_workers_str:
+if max_workers_str and int(max_workers_str) > 0:
     use_max_workers = int(max_workers_str)
-web_concurrency_str = os.getenv("WEB_CONCURRENCY", None)
-
 host = os.getenv("HOST", "0.0.0.0")
 port = os.getenv("PORT", "80")
 bind_env = os.getenv("BIND", None)
@@ -18,9 +18,8 @@
 cores = multiprocessing.cpu_count()
 workers_per_core = float(workers_per_core_str)
 default_web_concurrency = workers_per_core * cores
-if web_concurrency_str:
+if web_concurrency_str and int(web_concurrency_str) > 0:
     web_concurrency = int(web_concurrency_str)
-    assert web_concurrency > 0
 else:
     web_concurrency = max(int(default_web_concurrency), 2)
     if use_max_workers:
diff --git a/tests/test_start.py b/tests/test_start.py
index 27d9eab..88bf975 100644
--- a/tests/test_start.py
+++ b/tests/test_start.py
@@ -393,6 +393,50 @@ def test_start_server_uvicorn_gunicorn(
         )
         mock_logger.debug.assert_called_once_with("Running Uvicorn with Gunicorn.")  # type: ignore[attr-defined]  # noqa: E501
 
+    @pytest.mark.parametrize(
+        "app_module",
+        [
+            "inboard.app.base.main:app",
+            "inboard.app.fastapibase.main:app",
+            "inboard.app.starlettebase.main:app",
+        ],
+    )
+    def test_start_server_uvicorn_gunicorn_custom_config(
+        self,
+        app_module: str,
+        gunicorn_conf_path: Path,
+        logging_conf_dict: Dict[str, Any],
+        mock_logger: logging.Logger,
+        mocker: MockerFixture,
+        monkeypatch: MonkeyPatch,
+    ) -> None:
+        """Test `start.start_server` with Uvicorn managed by Gunicorn."""
+        assert os.getenv("GUNICORN_CONF", str(gunicorn_conf_path))
+        monkeypatch.setenv("LOG_FORMAT", "gunicorn")
+        monkeypatch.setenv("LOG_LEVEL", "debug")
+        monkeypatch.setenv("MAX_WORKERS", "1")
+        monkeypatch.setenv("PROCESS_MANAGER", "gunicorn")
+        monkeypatch.setenv("WEB_CONCURRENCY", "4")
+        assert os.getenv("LOG_FORMAT") == "gunicorn"
+        assert os.getenv("LOG_LEVEL") == "debug"
+        assert os.getenv("MAX_WORKERS") == "1"
+        assert os.getenv("PROCESS_MANAGER") == "gunicorn"
+        assert os.getenv("WEB_CONCURRENCY") == "4"
+        start.start_server(
+            str(os.getenv("PROCESS_MANAGER")),
+            app_module=app_module,
+            logger=mock_logger,
+            logging_conf_dict=logging_conf_dict,
+        )
+        monkeypatch.delenv("WEB_CONCURRENCY")
+        start.start_server(
+            str(os.getenv("PROCESS_MANAGER")),
+            app_module=app_module,
+            logger=mock_logger,
+            logging_conf_dict=logging_conf_dict,
+        )
+        mock_logger.debug.assert_called_with("Running Uvicorn with Gunicorn.")  # type: ignore[attr-defined]  # noqa: E501
+
     def test_start_server_uvicorn_incorrect_module(
         self,
         logging_conf_dict: Dict[str, Any],