diff --git a/argos/server/main.py b/argos/server/main.py
index 262f31a..0375e73 100644
--- a/argos/server/main.py
+++ b/argos/server/main.py
@@ -79,12 +79,11 @@ def setup_database(app):
     
     extra_settings = {}
     if settings.db_pool_size:
-        extra_settings.set("pool_size", settings.db_pool_size)
+        extra_settings.setdefault("pool_size", settings.db_pool_size)
 
     if settings.db_max_overflow:
-        extra_settings.set("max_overflow", settings.db_max_overflow)
+        extra_settings.setdefault("max_overflow", settings.db_max_overflow)
         
-    max_overflow=settings.db_max_overflow,
     engine = create_engine(
         settings.database_url,
         **extra_settings