SWE-bench

--- a/requests/sessions.py
+++ b/requests/sessions.py
@@ -412,6 +412,11 @@ def merge_setting(request_setting, session_setting, dict_class=OrderedDict):
     merged_setting = dict_class(to_key_val_list(session_setting))
     merged_setting.update(to_key_val_list(request_setting))
 
+    # Remove keys that are set to None. Extract the keys first to avoid
+    # mutating the dictionary while iterating over it.
+    none_keys = [k for (k, v) in merged_setting.items() if v is None]
+    for key in none_keys:
+        del merged_setting[key]
+
     return merged_setting