diff --git a/haystack/components/converters/openapi_functions.py b/haystack/components/converters/openapi_functions.py
index b35386003..8315807d8 100644
--- a/haystack/components/converters/openapi_functions.py
+++ b/haystack/components/converters/openapi_functions.py
@@ -48,16 +48,18 @@ class OpenAPIServiceToFunctions:
         openapi_imports.check()
 
     @component.output_types(documents=List[Document])
-    def run(self, sources: List[Union[str, Path, ByteStream]], system_messages: List[str]) -> Dict[str, Any]:
+    def run(
+        self, sources: List[Union[str, Path, ByteStream]], system_messages: Optional[List[str]] = None
+    ) -> Dict[str, Any]:
         """
         Processes OpenAPI specification URLs or files to extract functions that can be invoked via OpenAI function
-        calling mechanism. Each source is paired with a system message in one-to-one correspondence. The system message
-        is used to assist LLM in the response generation.
+        calling mechanism. Each source is paired with an optional system message. The system message can be potentially
+        used in LLM response generation.
 
         :param sources: A list of OpenAPI specification sources, which can be URLs, file paths, or ByteStream objects.
         :type sources: List[Union[str, Path, ByteStream]]
-        :param system_messages: A list of system messages corresponding to each source.
-        :type system_messages: List[str]
+        :param system_messages: A list of optional system messages corresponding to each source.
+        :type system_messages: Optional[List[str]]
         :return: A dictionary with a key 'documents' containing a list of Document objects. Each Document object
                  encapsulates a function definition and relevant metadata.
         :rtype: Dict[str, Any]
@@ -65,6 +67,7 @@ class OpenAPIServiceToFunctions:
         :raises ValueError: If the source type is not recognized or no functions are found in the OpenAPI specification.
         """
         documents: List[Document] = []
+        system_messages = system_messages or [""] * len(sources)
         for source, system_message in zip(sources, system_messages):
             openapi_spec_content = None
             if isinstance(source, (str, Path)):
@@ -83,14 +86,12 @@ class OpenAPIServiceToFunctions:
                 try:
                     service_openapi_spec = self._parse_openapi_spec(openapi_spec_content)
                     functions: List[Dict[str, Any]] = self._openapi_to_functions(service_openapi_spec)
-                    docs = [
-                        Document(
-                            content=json.dumps(function),
-                            meta={"spec": service_openapi_spec, "system_message": system_message},
-                        )
-                        for function in functions
-                    ]
-                    documents.extend(docs)
+                    for function in functions:
+                        meta: Dict[str, Any] = {"spec": service_openapi_spec}
+                        if system_message:
+                            meta["system_message"] = system_message
+                        doc = Document(content=json.dumps(function), meta=meta)
+                        documents.append(doc)
                 except Exception as e:
                     logger.error("Error processing OpenAPI specification from source %s: %s", source, e)
 
diff --git a/test/components/converters/test_openapi_functions.py b/test/components/converters/test_openapi_functions.py
index 79e202ee6..db704ae6a 100644
--- a/test/components/converters/test_openapi_functions.py
+++ b/test/components/converters/test_openapi_functions.py
@@ -219,3 +219,23 @@ class TestOpenAPIServiceToFunctions:
             # check that the metadata is as expected
             assert doc.meta["system_message"] == "Some system message we don't care about here"
             assert doc.meta["spec"] == json.loads(json_serperdev_openapi_spec)
+
+    def test_run_with_file_source_and_none_system_messages(self, json_serperdev_openapi_spec):
+        service = OpenAPIServiceToFunctions()
+        spec_stream = ByteStream.from_string(json_serperdev_openapi_spec)
+
+        # we now omit the system_messages argument
+        result = service.run(sources=[spec_stream])
+        assert len(result["documents"]) == 1
+        doc = result["documents"][0]
+
+        # check that the content is as expected
+        assert (
+            doc.content
+            == '{"name": "search", "description": "Search the web with Google", "parameters": {"type": "object", '
+            '"properties": {"requestBody": {"type": "object", "properties": {"q": {"type": "string"}}}}}}'
+        )
+
+        # check that the metadata is as expected, system_message should not be present
+        assert "system_message" not in doc.meta
+        assert doc.meta["spec"] == json.loads(json_serperdev_openapi_spec)