diff --git a/app/controllers/admin/product_import_controller.rb b/app/controllers/admin/product_import_controller.rb
index 9331023aad..7cbb8f64b5 100644
--- a/app/controllers/admin/product_import_controller.rb
+++ b/app/controllers/admin/product_import_controller.rb
@@ -30,13 +30,13 @@ module Admin
     def validate_data
       return unless process_data('validate')
 
-      render json: @importer.import_results, response: 200
+      render json: @importer.import_results
     end
 
     def save_data
       return unless process_data('save')
 
-      render json: @importer.save_results, response: 200
+      render json: @importer.save_results
     end
 
     def reset_absent_products
@@ -76,7 +76,7 @@ module Admin
       begin
         @importer.public_send("#{method}_entries")
       rescue StandardError => e
-        render json: e.message, response: 500
+        render plain: e.message, status: :internal_server_error
         return false
       end
 
diff --git a/app/models/product_import/entry_validator.rb b/app/models/product_import/entry_validator.rb
index f720af4aa5..bc1c4153ce 100644
--- a/app/models/product_import/entry_validator.rb
+++ b/app/models/product_import/entry_validator.rb
@@ -79,10 +79,9 @@ module ProductImport
         if entry.attributes['on_hand'].present?
           new_variant.on_hand = entry.attributes['on_hand']
         end
+        check_on_hand_nil(entry, new_variant)
       end
 
-      check_on_hand_nil(entry, new_variant)
-
       if new_variant.valid?
         entry.product_object = new_variant
         entry.validates_as = 'new_variant' unless entry.errors?
diff --git a/spec/models/product_importer_spec.rb b/spec/models/product_importer_spec.rb
index a2a2917f20..a2a5b96b25 100644
--- a/spec/models/product_importer_spec.rb
+++ b/spec/models/product_importer_spec.rb
@@ -1012,6 +1012,30 @@ RSpec.describe ProductImport::ProductImporter do
       expect(lettuce.count_on_hand).to eq 96   # In different enterprise; unchanged
     end
   end
+
+  # Fix https://github.com/openfoodfoundation/openfoodnetwork/issues/12973
+  describe 'update existing product with units and on_hand/on_demand is empty' do
+    let(:csv_data) do
+      CSV.generate do |csv|
+        csv << ["name", "producer", "category", "on_hand", "price", "units", "unit_type",
+                "shipping_category", "on_demand"]
+        csv << ["Beetroot", enterprise3.name, "Vegetables", "", "6.50", "", "g",
+                shipping_category.name, "1"]
+      end
+    end
+    let(:importer) { import_data csv_data }
+
+    it "returns the units error due to invalid data" do
+      importer.validate_entries
+      entries = JSON.parse(importer.entries_json)
+
+      expect(entries.dig('2', 'errors')).to eq(
+        {
+          "units" => "Units can't be blank",
+        }
+      )
+    end
+  end
 end
 
 private