diff --git a/nextpipe/flow.py b/nextpipe/flow.py
index 5d8329f..a4bc308 100644
--- a/nextpipe/flow.py
+++ b/nextpipe/flow.py
@@ -133,7 +133,7 @@ def _get_inputs(self, node: DAGNode) -> List[object]:
         )
 
     @staticmethod
-    def __run_node(node: DAGNode, inputs: List[object], client: Client) -> List[object] | object | None:
+    def __run_node(node: DAGNode, inputs: List[object], client: Client) -> Union[List[object], object, None]:
         utils.log(f"Running node {node.step.get_name()}")
 
         # Skip the node if it is optional and the condition is not met