diff --git a/schemas/article/RawArticle.ts b/schemas/article/RawArticle.ts
index 6c956e5..99eae03 100644
--- a/schemas/article/RawArticle.ts
+++ b/schemas/article/RawArticle.ts
@@ -8,7 +8,10 @@ import { getRandomNumberAsString } from '@/functions/utils/randomDefaultValues'
  */
 export const RawArticleSchema = z.object({
   article_id: z.string().default(getRandomNumberAsString()).optional(),
-  article_category_id: z.number().optional(),
+  article_category_id: z
+    .number()
+    .transform((val) => (val === null ? undefined : val))
+    .optional(),
   article_name: z.string().default('Article-XY'),
   article_code: z.string().default(getRandomNumberAsString()).optional(),
   article_eanCode: z.string().default(getRandomNumberAsString()).optional(),
diff --git a/schemas/utils/schemaDefaults.ts b/schemas/utils/schemaDefaults.ts
index 93a676b..1513f26 100644
--- a/schemas/utils/schemaDefaults.ts
+++ b/schemas/utils/schemaDefaults.ts
@@ -79,6 +79,9 @@ export default function schemaDefaults<Schema extends z.ZodFirstPartySchemaTypes
     case z.ZodFirstPartyTypeKind.ZodCatch:
       return schema._def.catchValue.call(null, {} as any)
 
+    case z.ZodFirstPartyTypeKind.ZodEffects:
+      return schemaDefaults((schema as any)._def.schema, options)
+
     default:
       throw new Error(`Unsupported type ${(schema as any)._def.typeName}`)
   }
diff --git a/tests/api/Get.test.ts b/tests/api/Get.test.ts
index c6ab996..d999516 100644
--- a/tests/api/Get.test.ts
+++ b/tests/api/Get.test.ts
@@ -3,6 +3,7 @@ import GET from '@/api/GET'
 import { setAuthorization } from '@/config/authorization'
 import { RawInvoices, safeParseRawInvoices } from '@/schemas/invoice/RawInvoices'
 import { RawArticles, safeParseRawArticles } from '@/schemas/article/RawArticles'
+import { RawCustomers, safeParseRawCustomers } from '@/schemas/customer/RawCustomers'
 
 beforeEach(() => {
   setAuthorization(process.env.AUTH_TOKEN!)
@@ -15,14 +16,20 @@ describe('Testing #GET function: ', () => {
     expect(safeParseRawInvoices(response).success).toBe(true)
   })
 
-  test('articles endpoint should return RawInvoices object', async () => {
-    const response = await GET<RawArticles>('articles')
+  test('articles endpoint should return RawArticles object', async () => {
+    const response = await GET<RawArticles>('articles', [`limit=${5}`])
 
     expect(safeParseRawArticles(response).success).toBe(true)
+    expect(response.articles.length).toBeGreaterThanOrEqual(5)
   })
 
   test('calling without filter params should yield results anyway', async () => {
     const articles = await GET<RawArticles>('articles')
     expect(articles.articles.length).toBeGreaterThan(0)
   })
+
+  test('validity of customer schema against response of /users endpoint', async () => {
+    const customers = await GET<RawCustomers>('users')
+    expect(safeParseRawCustomers(customers).success).toBe(true)
+  })
 })
diff --git a/tests/invoices/getInvoices.test.ts b/tests/invoices/getInvoices.test.ts
index ab734b6..cc9d21a 100644
--- a/tests/invoices/getInvoices.test.ts
+++ b/tests/invoices/getInvoices.test.ts
@@ -13,25 +13,20 @@ test('getInvoices - limit of 10, Expect 10 invoices', async () => {
   expect(invoices.length).toBeLessThanOrEqual(10)
 })
 
-test('getInvoices - limit of 0, Expect 0 invoices', async () => {
-  const invoices = await getInvoices(0)
-  expect(invoices).toHaveLength(0)
-})
-
 test(
-  'getInvoices - limit of 1000, Expect at least 0 invoices',
+  'getInvoices - limit of 1000 expect at least 0 invoices',
   async () => {
     const invoices = await getInvoices(1000)
     expect(Array.isArray(invoices)).toBe(true)
-    expect(invoices.length).toBeGreaterThan(0)
+    expect(invoices.length).toBeGreaterThanOrEqual(0)
   },
   10 * 1000,
 )
 
-test('getInvoices - limit of -1, Expect at least 0 invoices', async () => {
-  const invoices = await getInvoices(-1)
+test('getInvoices - default limit expect array of invoices', async () => {
+  const invoices = await getInvoices()
   expect(Array.isArray(invoices)).toBe(true)
-  expect(invoices.length).toBeGreaterThan(0)
+  expect(invoices.length).toBeGreaterThanOrEqual(0)
 })
 
 test('getInvoices - No Authentication set; Expect Authentication Failure', async () => {