Checkout Totals Sort Order fields can not be empty and should be a number.

Author: barbanet

app/code/Magento/Sales/etc/adminhtml/system.xml

@@ -27,18 +27,23 @@
                 <label>Checkout Totals Sort Order</label>
                 <field id="discount" translate="label" type="text" sortOrder="2" showInDefault="1" showInWebsite="1" showInStore="0" canRestore="1">
                     <label>Discount</label>
+                    <validate>required-number validate-number</validate>
                 </field>
                 <field id="grand_total" translate="label" type="text" sortOrder="5" showInDefault="1" showInWebsite="1" showInStore="0" canRestore="1">
                     <label>Grand Total</label>
+                    <validate>required-number validate-number</validate>
                 </field>
                 <field id="shipping" translate="label" type="text" sortOrder="3" showInDefault="1" showInWebsite="1" showInStore="0" canRestore="1">
                     <label>Shipping</label>
+                    <validate>required-number validate-number</validate>
                 </field>
                 <field id="subtotal" translate="label" type="text" sortOrder="1" showInDefault="1" showInWebsite="1" showInStore="0" canRestore="1">
                     <label>Subtotal</label>
+                    <validate>required-number validate-number</validate>
                 </field>
                 <field id="tax" translate="label" type="text" sortOrder="4" showInDefault="1" showInWebsite="1" showInStore="0" canRestore="1">
                     <label>Tax</label>
+                    <validate>required-number validate-number</validate>
                 </field>
             </group>
             <group id="reorder" translate="label" type="text" sortOrder="20" showInDefault="1" showInWebsite="1" showInStore="1">

app/code/Magento/Weee/etc/adminhtml/system.xml

@@ -44,6 +44,7 @@
             <group id="totals_sort">
                 <field id="weee" translate="label" type="text" sortOrder="4" showInDefault="1" showInWebsite="1" showInStore="0" canRestore="1">
                     <label>Fixed Product Tax</label>
+                    <validate>required-number validate-number</validate>
                 </field>
             </group>
         </section>