diff --git a/substrafl/nodes/aggregation_node.py b/substrafl/nodes/aggregation_node.py
index e39ca2a5..80988733 100644
--- a/substrafl/nodes/aggregation_node.py
+++ b/substrafl/nodes/aggregation_node.py
@@ -77,7 +77,7 @@ def update_states(
             else None
         )
 
-        task_metadata = {"round_idx": round_idx} if round_idx is not None else {}
+        task_metadata = {"round_idx": str(round_idx)} if round_idx is not None else {}
         aggregate_task = substra.schemas.ComputePlanTaskSpec(
             function_key=str(uuid.uuid4()),  # bogus function key
             task_id=op_id,
diff --git a/substrafl/nodes/test_data_node.py b/substrafl/nodes/test_data_node.py
index 45001de3..32c6403f 100644
--- a/substrafl/nodes/test_data_node.py
+++ b/substrafl/nodes/test_data_node.py
@@ -114,7 +114,7 @@ def update_states(
                 parent_task_output_identifier=OutputIdentifiers.predictions,
             )
         ]
-        task_metadata = {"round_idx": round_idx} if round_idx is not None else {}
+        task_metadata = {"round_idx": str(round_idx)} if round_idx is not None else {}
 
         predicttask = substra.schemas.ComputePlanTaskSpec(
             function_key=str(uuid.uuid4()),  # bogus function key
diff --git a/substrafl/nodes/train_data_node.py b/substrafl/nodes/train_data_node.py
index 83e48f98..5e248435 100644
--- a/substrafl/nodes/train_data_node.py
+++ b/substrafl/nodes/train_data_node.py
@@ -66,7 +66,7 @@ def init_states(
                 ),
             },
             metadata={
-                "round_idx": round_idx,
+                "round_idx": str(round_idx),
             },
             tag=TaskType.INITIALIZATION,
             worker=self.organization_id,
@@ -160,7 +160,7 @@ def update_states(
         else:
             shared_inputs = []
 
-        task_metadata = {"round_idx": round_idx} if round_idx is not None else {}
+        task_metadata = {"round_idx": str(round_idx)} if round_idx is not None else {}
         train_task = substra.schemas.ComputePlanTaskSpec(
             function_key=str(uuid.uuid4()),  # bogus function key
             task_id=op_id,