diff --git a/mistral/engine/default_engine.py b/mistral/engine/default_engine.py
index 4a9123f21..8fbe03c5d 100644
--- a/mistral/engine/default_engine.py
+++ b/mistral/engine/default_engine.py
@@ -19,6 +19,8 @@ from oslo_config import cfg
 from oslo_log import log as logging
 from osprofiler import profiler
 
+from mistral_lib import actions as ml_actions
+
 from mistral.db import utils as db_utils
 from mistral.db.v2 import api as db_api
 from mistral.db.v2.sqlalchemy import models as db_models
@@ -145,6 +147,12 @@ class DefaultEngine(base.Engine):
         with db_api.transaction():
             if wf_action:
                 action_ex = db_api.get_workflow_execution(action_ex_id)
+
+                # If result is None it means that it's a normal subworkflow
+                # output and we just need to fetch it from the model.
+                # This is just an optimization to not send data over RPC
+                if result is None:
+                    result = ml_actions.Result(data=action_ex.output)
             else:
                 action_ex = db_api.get_action_execution(action_ex_id)
 
diff --git a/mistral/engine/engine_server.py b/mistral/engine/engine_server.py
index a76b2aadf..776469bb6 100644
--- a/mistral/engine/engine_server.py
+++ b/mistral/engine/engine_server.py
@@ -153,7 +153,7 @@ class EngineServer(service_base.MistralService):
             "Received RPC request 'on_action_complete'[action_ex_id=%s, "
             "result=%s]",
             action_ex_id,
-            result.cut_repr()
+            result.cut_repr() if result else '<unknown>'
         )
 
         return self.engine.on_action_complete(action_ex_id, result, wf_action)
diff --git a/mistral/engine/workflows.py b/mistral/engine/workflows.py
index f0402741b..bdd4efee7 100644
--- a/mistral/engine/workflows.py
+++ b/mistral/engine/workflows.py
@@ -551,7 +551,9 @@ class Workflow(object):
 
     def _send_result_to_parent_workflow(self):
         if self.wf_ex.state == states.SUCCESS:
-            result = ml_actions.Result(data=self.wf_ex.output)
+            # The result of the sub workflow is already saved
+            # so there's no need to send it over RPC.
+            result = None
         elif self.wf_ex.state == states.ERROR:
             err_msg = (
                 self.wf_ex.state_info or