diff --git a/src/smolagents/agents.py b/src/smolagents/agents.py
index 1c5534565..b31570881 100644
--- a/src/smolagents/agents.py
+++ b/src/smolagents/agents.py
@@ -379,6 +379,7 @@ def _setup_managed_agents(self, managed_agents: list | None = None) -> None:
                     "additional_args": {
                         "type": "object",
                         "description": "Dictionary of extra inputs to pass to the managed agent, e.g. images, dataframes, or any other contextual data it may need.",
+                        "nullable": True,
                     },
                 }
                 agent.output_type = "string"