diff --git a/base_classes/NXmonochromator.nxdl.xml b/base_classes/NXmonochromator.nxdl.xml
index 5375193f19..2ae776e487 100644
--- a/base_classes/NXmonochromator.nxdl.xml
+++ b/base_classes/NXmonochromator.nxdl.xml
@@ -65,7 +65,7 @@
              Energy dispersion at the exit slit.
         </doc>
     </field>
-    <field name="wavelength_dispersion" type="NX_FLOAT" units="NX_ANY">
+    <field name="wavelength_dispersion" type="NX_FLOAT" units="nm/mm">
         <doc>
              Wavelength dispersion at the exit slit.
         </doc>