diff --git a/applications/NXmx.nxdl.xml b/applications/NXmx.nxdl.xml
index 08a8f5e08c..554396e609 100644
--- a/applications/NXmx.nxdl.xml
+++ b/applications/NXmx.nxdl.xml
@@ -858,7 +858,7 @@
                             <item value="rectangular"/>
                         </enumeration>
                     </field>
-                    <field name="incident_polarisation_stokes" type="NX_NUMBER" recommended="false"
+                    <field name="incident_polarisation_stokes" type="NX_NUMBER" optional="true"
                         deprecated="use incident_polarization_stokes, see https://github.com/nexusformat/definitions/issues/708">
                         <doc>
                             Polarization vector on entering beamline