diff --git a/apps/edr-freight-api/src/modules/rule-engine/services/rate-change-requests.service.spec.ts b/apps/edr-freight-api/src/modules/rule-engine/services/rate-change-requests.service.spec.ts
index be7c8984c..f8617cd65 100644
--- a/apps/edr-freight-api/src/modules/rule-engine/services/rate-change-requests.service.spec.ts
+++ b/apps/edr-freight-api/src/modules/rule-engine/services/rate-change-requests.service.spec.ts
@@ -118,6 +118,19 @@ describe('RateChangeRequestsService', () => {
       expect(request.payload).toEqual({ destinationYardId: 'yard-c' });
     });
 
+    it('carries baseLiters — a switch to PER_LITER keeps its billing base', async () => {
+      const { service } = build({
+        rate: liveRate({ rateUnit: 'PER_WAGON', baseLiters: null }),
+      });
+
+      const request = await service.submit({
+        rateId: 'rate-1',
+        update: { rateUnit: 'PER_LITER', baseLiters: 3 },
+      });
+
+      expect(request.payload).toEqual({ rateUnit: 'PER_LITER', baseLiters: 3 });
+    });
+
     it('rejects a no-op — 100 posted against a live 100.0000 is not a change', async () => {
       const { service } = build();
       await expect(
diff --git a/apps/edr-freight-api/src/modules/rule-engine/services/rate-change-requests.service.ts b/apps/edr-freight-api/src/modules/rule-engine/services/rate-change-requests.service.ts
index 8cc97f343..8913c9ef9 100644
--- a/apps/edr-freight-api/src/modules/rule-engine/services/rate-change-requests.service.ts
+++ b/apps/edr-freight-api/src/modules/rule-engine/services/rate-change-requests.service.ts
@@ -42,6 +42,10 @@ const DIFFABLE_FIELDS = [
   // LIVE last-mile rate would diff to "nothing changed".
   'minKm',
   'maxKm',
+  // PER_LITER fuel surcharge billing base. Missing here, a switch to PER_LITER
+  // dropped the submitted liters and validation failed with "needs a base
+  // liters amount" even though the payload carried one.
+  'baseLiters',
 ] as const;
 
 /**