diff --git a/mesa_llm/module_llm.py b/mesa_llm/module_llm.py
index 52a18fa..13a5382 100644
--- a/mesa_llm/module_llm.py
+++ b/mesa_llm/module_llm.py
@@ -77,10 +77,8 @@ def get_messages(self, prompt: str | list[str]) -> list[dict]:
         """
         messages = []
 
-        # Always include a system message. If no prompt provided, set content to None regardless of system_prompt
-        system_content = (
-            None if not prompt else (self.system_prompt if self.system_prompt else None)
-        )
+        # Always include a system message. Default to empty string if no system prompt to support Ollama
+        system_content = self.system_prompt if self.system_prompt else ""
         messages.append({"role": "system", "content": system_content})
 
         if prompt: