diff --git a/pydantic_ai_slim/pydantic_ai/messages.py b/pydantic_ai_slim/pydantic_ai/messages.py
index 5c61ce0a..d461b7ff 100644
--- a/pydantic_ai_slim/pydantic_ai/messages.py
+++ b/pydantic_ai_slim/pydantic_ai/messages.py
@@ -32,7 +32,7 @@ class DynamicSystemPromptPart(SystemPromptPart):
     Same as `SystemPromptPart`, but its content is regenerated on each run.
     """
 
-    ref: int | None = None
+    ref: str
     """The ref ID of the system prompt function that generated this part."""