diff --git a/ghost/members-api/lib/controllers/RouterController.js b/ghost/members-api/lib/controllers/RouterController.js
index b5af1d8599..3ef803d5a0 100644
--- a/ghost/members-api/lib/controllers/RouterController.js
+++ b/ghost/members-api/lib/controllers/RouterController.js
@@ -10,6 +10,7 @@ const messages = {
     notFound: 'Not Found.',
     offerNotFound: 'This offer does not exist.',
     offerArchived: 'This offer is archived.',
+    tierNotFound: 'This tier does not exist.',
     tierArchived: 'This tier is archived.',
     existingSubscription: 'A subscription exists for this Member.',
     unableToCheckout: 'Unable to initiate checkout session',
@@ -258,9 +259,22 @@ module.exports = class RouterController {
 
             tier = await this._tiersService.api.read(offer.tier.id);
             cadence = offer.cadence;
-        } else {
+        } else if (tierId) {
             offer = null;
-            tier = await this._tiersService.api.read(tierId);
+
+            try {
+                // If the tierId is not a valid ID, the following line will throw
+                tier = await this._tiersService.api.read(tierId);
+
+                if (!tier) {
+                    throw undefined;
+                }
+            } catch (err) {
+                throw new BadRequestError({
+                    message: tpl(messages.tierNotFound),
+                    context: 'Tier with id "' + tierId + '" not found'
+                });
+            }
         }
 
         if (tier.status === 'archived') {
diff --git a/ghost/members-api/test/unit/lib/controllers/router.test.js b/ghost/members-api/test/unit/lib/controllers/router.test.js
index ef1d5dd7fb..cd85e09321 100644
--- a/ghost/members-api/test/unit/lib/controllers/router.test.js
+++ b/ghost/members-api/test/unit/lib/controllers/router.test.js
@@ -162,7 +162,7 @@ describe('RouterController', function () {
                 }
             });
 
-            it('returns a BadRequestError if offer is not found', async function () {
+            it('returns a BadRequestError if offer is not found by offerId', async function () {
                 offersAPI = {
                     getOffer: sinon.stub().resolves(null)
                 };
@@ -184,6 +184,56 @@ describe('RouterController', function () {
                     assert.equal(error.context, 'Offer with id "invalid" not found');
                 }
             });
+
+            it('returns a BadRequestError if tier is not found by tierId', async function () {
+                tiersService = {
+                    api: {
+                        read: sinon.stub().resolves(null)
+                    }
+                };
+
+                const routerController = new RouterController({
+                    tiersService,
+                    paymentsService,
+                    offersAPI,
+                    stripeAPIService,
+                    labsService
+                });
+
+                try {
+                    await routerController._getSubscriptionCheckoutData({tierId: 'invalid', cadence: 'year'});
+
+                    assert.fail('Expected function to throw BadRequestError');
+                } catch (error) {
+                    assert(error instanceof errors.BadRequestError, 'Error should be an instance of BadRequestError');
+                    assert.equal(error.context, 'Tier with id "invalid" not found');
+                }
+            });
+
+            it('returns a BadRequestError if fetching tier by tierId throws', async function () {
+                tiersService = {
+                    api: {
+                        read: sinon.stub().rejects(new Error('Fail to fetch tier'))
+                    }
+                };
+
+                const routerController = new RouterController({
+                    tiersService,
+                    paymentsService,
+                    offersAPI,
+                    stripeAPIService,
+                    labsService
+                });
+
+                try {
+                    await routerController._getSubscriptionCheckoutData({tierId: 'invalid', cadence: 'year'});
+
+                    assert.fail('Expected function to throw BadRequestError');
+                } catch (error) {
+                    assert(error instanceof errors.BadRequestError, 'Error should be an instance of BadRequestError');
+                    assert.equal(error.context, 'Tier with id "invalid" not found');
+                }
+            });
         });
 
         afterEach(function () {