diff --git a/api/core/workflow/nodes/list_operator/node.py b/api/core/workflow/nodes/list_operator/node.py
index 4ccafb6fb1..b91fc622f6 100644
--- a/api/core/workflow/nodes/list_operator/node.py
+++ b/api/core/workflow/nodes/list_operator/node.py
@@ -184,12 +184,11 @@ class ListOperatorNode(BaseNode):
         value = int(self.graph_runtime_state.variable_pool.convert_template(self._node_data.extract_by.serial).text)
         if value < 1:
             raise ValueError(f"Invalid serial index: must be >= 1, got {value}")
+        if value > len(variable.value):
+            raise InvalidKeyError(f"Invalid serial index: must be <= {len(variable.value)}, got {value}")
         value -= 1
-        if len(variable.value) > int(value):
-            result = [variable.value[value]]
-        else:
-            result = []
-        return variable.model_copy(update={"value": result})
+        result = variable.value[value]
+        return variable.model_copy(update={"value": [result]})
 
 
 def _get_file_extract_number_func(*, key: str) -> Callable[[File], int]: