diff --git a/flask_request_id_header/middleware/__init__.py b/flask_request_id_header/middleware/__init__.py
index fd10268..25e0b2d 100644
--- a/flask_request_id_header/middleware/__init__.py
+++ b/flask_request_id_header/middleware/__init__.py
@@ -34,7 +34,7 @@ def __init__(self, app: App):
         :param app: Flask application
         """
         self.app = app.wsgi_app
-        self._unique_request_id_value_prefix = app.config['REQUEST_ID_UNIQUE_VALUE_PREFIX']
+        self._unique_request_id_value_prefix = app.config.get('REQUEST_ID_UNIQUE_VALUE_PREFIX')
         self._header_name = "X-Request-ID"
         self._flask_header_name = f"HTTP_{ self._header_name.upper().replace('-', '_') }"