diff --git a/CHANGELOG b/CHANGELOG
index 6a04dbf042..3c85f34e20 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,5 @@
+* Allow setting unit on request without product
+
 Version 5.4.0 - 2019-11-04
 * Bug fixes (see mercurial logs for details)
 
diff --git a/purchase_request.py b/purchase_request.py
index 1a446adc05..8041efe7d6 100644
--- a/purchase_request.py
+++ b/purchase_request.py
@@ -34,7 +34,9 @@ class PurchaseRequest(ModelSQL, ModelView):
     uom = fields.Many2One('product.uom', 'UOM', select=True,
         ondelete='RESTRICT',
         domain=[
-            ('category', '=', Eval('product_uom_category')),
+            If(Bool(Eval('product_uom_category')),
+                ('category', '=', Eval('product_uom_category')),
+                ('category', '!=', -1)),
             ],
         states={
             'required': Bool(Eval('product')),