diff --git a/contributed_definitions/NXopt.nxdl.xml b/contributed_definitions/NXopt.nxdl.xml
index 254b4a21d..801de867e 100644
--- a/contributed_definitions/NXopt.nxdl.xml
+++ b/contributed_definitions/NXopt.nxdl.xml
@@ -408,7 +408,7 @@ optical spectroscopy experiments-->
                                  If the parameter_type is `other` a name should be specified here.
                             </doc>
                         </field>
-                        <field name="number_of_parameters" type="NX_UINT" units="NX_UNITLESS">
+                        <field name="number_of_parameters" type="NX_POSINT" units="NX_UNITLESS">
                             <doc>
                                  Number of different parameter values at which the measurement
                                  was performed. For example, if the measurement was performed at
diff --git a/contributed_definitions/nyaml/NXopt.yaml b/contributed_definitions/nyaml/NXopt.yaml
index 447373da3..15289ceb7 100644
--- a/contributed_definitions/nyaml/NXopt.yaml
+++ b/contributed_definitions/nyaml/NXopt.yaml
@@ -298,7 +298,7 @@ NXopt:
               doc: |
                 If the parameter_type is `other` a name should be specified here.
               exists: optional
-            number_of_parameters(NX_UINT):
+            number_of_parameters(NX_POSINT):
               doc: |
                 Number of different parameter values at which the measurement
                 was performed. For example, if the measurement was performed at