diff --git a/plugins/CuraEngineBackend/StartSliceJob.py b/plugins/CuraEngineBackend/StartSliceJob.py
index 7fd703f8ca..4d94e5e8a9 100644
--- a/plugins/CuraEngineBackend/StartSliceJob.py
+++ b/plugins/CuraEngineBackend/StartSliceJob.py
@@ -81,14 +81,14 @@ class StartSliceJob(Job):
 
         self._sendSettings(self._profile)
 
-        slice_message = Cura_pb2.Slice()
+        slice_message = self._socket.createMessage("cura.proto.Slice");
 
         for group in object_groups:
-            group_message = slice_message.object_lists.add()
+            group_message = slice_message.addRepeatedMessage("object_lists");
             for object in group:
                 mesh_data = object.getMeshData().getTransformed(object.getWorldTransformation())
 
-                obj = group_message.objects.add()
+                obj = group_message.addRepeatedMessage("objects");
                 obj.id = id(object)
 
                 verts = numpy.array(mesh_data.getVertices())
@@ -115,13 +115,13 @@ class StartSliceJob(Job):
             return str(value).encode("utf-8")
 
     def _sendSettings(self, profile):
-        msg = Cura_pb2.SettingList()
+        msg = self._socket.createMessage("cura.proto.SettingList");
         settings = profile.getAllSettingValues(include_machine = True)
         start_gcode = settings["machine_start_gcode"]
         settings["material_bed_temp_prepend"] = "{material_bed_temperature}" not in start_gcode
         settings["material_print_temp_prepend"] = "{material_print_temperature}" not in start_gcode
         for key, value in settings.items():
-            s = msg.settings.add()
+            s = msg.addRepeatedMessage("settings")
             s.name = key
             if key == "machine_start_gcode" or key == "machine_end_gcode":
                 s.value = self._expandGcodeTokens(key, value, settings)
@@ -134,7 +134,7 @@ class StartSliceJob(Job):
         profile = node.callDecoration("getProfile")
         if profile:
             for key, value in profile.getAllSettingValues().items():
-                setting = message.settings.add()
+                setting = message.addRepeatedMessage("settings")
                 setting.name = key
                 setting.value = str(value).encode()
 
@@ -145,7 +145,7 @@ class StartSliceJob(Job):
             return
 
         for key, value in object_settings.items():
-            setting = message.settings.add()
+            setting = message.addRepeatedMessage("settings")
             setting.name = key
             setting.value = str(value).encode()