diff --git a/onnx/test/inference_function_test.py b/onnx/test/inference_function_test.py
index cf6d6fdb9cc..fc2c196a514 100644
--- a/onnx/test/inference_function_test.py
+++ b/onnx/test/inference_function_test.py
@@ -6,7 +6,8 @@
 
 
 def call_infer_types(
-    schema: onnx.defs.OpSchema, inputs: Iterable[onnx.TypeProto], num_outputs: int, node: Optional[onnx.NodeProto] = None
+    schema: onnx.defs.OpSchema, inputs: Iterable[onnx.TypeProto],
+    num_outputs: Optional[int] = None, node: Optional[onnx.NodeProto] = None
 ) -> Optional[List[onnx.TypeProto]]:
     inputs = list(inputs)
 
@@ -14,6 +15,8 @@ def call_infer_types(
         return None
 
     if node is None:
+        if num_outputs is None:
+            raise ValueError("Either node or num_outputs must be specified.")
         input_names = [f"in{i}" for i in range(len(inputs))]
         output_names = [f"out{i}" for i in range(num_outputs)]
         node = onnx.helper.make_node(schema.name, input_names, output_names)