diff --git a/src/test/java/com/tabs/dao/DAOEdgeCaseTest.java b/src/test/java/com/tabs/dao/DAOEdgeCaseTest.java
new file mode 100644
index 0000000..c34e4cb
--- /dev/null
+++ b/src/test/java/com/tabs/dao/DAOEdgeCaseTest.java
@@ -0,0 +1,135 @@
+package com.tabs.dao;
+
+import com.tabs.models.Customer;
+import com.tabs.models.Invoice;
+import com.tabs.models.Plan;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.lang.reflect.Field;
+import java.time.LocalDate;
+import java.time.YearMonth;
+import java.util.List;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class DAOEdgeCaseTest {
+
+    @BeforeEach
+    void resetSharedState() throws Exception {
+        clearStaticMap(BillingDAOImpl.class, "invoices");
+        clearStaticMap(CustomerDAOImpl.class, "customers");
+    }
+
+    @Test
+    void billingAddInvoiceIgnoresNullAndMissingIdAndGettersReturnSafeDefaults() {
+        BillingDAO billingDAO = new BillingDAOImpl();
+
+        assertDoesNotThrow(() -> billingDAO.addInvoice(null));
+        assertDoesNotThrow(() -> billingDAO.addInvoice(new Invoice()));
+
+        assertNull(billingDAO.getInvoiceById(null));
+        assertNull(billingDAO.getInvoiceById("missing"));
+        assertTrue(billingDAO.getInvoicesByCustomer(null).isEmpty());
+        assertTrue(billingDAO.getInvoicesBySubscriptionAndPeriod(null, YearMonth.of(2024, 1)).isEmpty());
+        assertTrue(billingDAO.getInvoicesBySubscriptionAndPeriod("sub-1", null).isEmpty());
+        assertTrue(billingDAO.getAllInvoices().isEmpty());
+    }
+
+    @Test
+    void billingReturnsMatchingInvoiceAndFiltersBySubscriptionAndPeriod() {
+        BillingDAO billingDAO = new BillingDAOImpl();
+
+        Invoice invoice = new Invoice();
+        invoice.setInvoiceId("inv-1");
+        invoice.setCustId("cust-1");
+        invoice.setSubscriptionId("sub-1");
+        invoice.setBillingDate(LocalDate.of(2024, 1, 15));
+        billingDAO.addInvoice(invoice);
+
+        assertSame(invoice, billingDAO.getInvoiceById("inv-1"));
+        assertEquals(1, billingDAO.getInvoicesByCustomer("cust-1").size());
+        assertEquals(invoice, billingDAO.getInvoicesBySubscriptionAndPeriod("sub-1", YearMonth.of(2024, 1)).get(0));
+        assertTrue(billingDAO.getInvoicesByCustomer("other").isEmpty());
+        assertTrue(billingDAO.getInvoicesBySubscriptionAndPeriod("sub-1", YearMonth.of(2024, 2)).isEmpty());
+    }
+
+    @Test
+    void billingUpdateIgnoresInvalidInputsAndReplacesExistingInvoice() {
+        BillingDAO billingDAO = new BillingDAOImpl();
+
+        Invoice original = new Invoice();
+        original.setInvoiceId("inv-1");
+        original.setCustId("cust-1");
+        original.setSubscriptionId("sub-1");
+        original.setBillingDate(LocalDate.of(2024, 1, 1));
+        billingDAO.addInvoice(original);
+
+        Invoice updated = new Invoice();
+        updated.setInvoiceId("inv-1");
+        updated.setCustId("cust-2");
+        updated.setSubscriptionId("sub-2");
+        updated.setBillingDate(LocalDate.of(2024, 2, 1));
+        assertDoesNotThrow(() -> billingDAO.updateInvoice(updated));
+        assertSame(updated, billingDAO.getInvoiceById("inv-1"));
+        assertEquals(1, billingDAO.getAllInvoices().size());
+
+        assertDoesNotThrow(() -> billingDAO.updateInvoice(null));
+        assertEquals(1, billingDAO.getAllInvoices().size());
+
+        Invoice noId = new Invoice();
+        assertDoesNotThrow(() -> billingDAO.updateInvoice(noId));
+        assertEquals(1, billingDAO.getAllInvoices().size());
+    }
+
+    @Test
+    void customerAddGetDeleteHandleNullAndMissingInputsSafely() {
+        CustomerDAO customerDAO = new CustomerDAOImpl();
+
+        assertDoesNotThrow(() -> customerDAO.addCustomer(null));
+        assertDoesNotThrow(() -> customerDAO.deleteCustomer(null));
+        assertNull(customerDAO.getCustomerById(null));
+        assertNull(customerDAO.getCustomerById("missing"));
+        assertTrue(customerDAO.getAllCustomers().isEmpty());
+    }
+
+    @Test
+    void customerUpdateReplacesExistingCustomerAndIgnoresInvalidInputs() {
+        CustomerDAO customerDAO = new CustomerDAOImpl();
+
+        Customer customer = new Customer();
+        customer.setCustId("cust-1");
+        customerDAO.addCustomer(customer);
+
+        Customer updated = new Customer();
+        updated.setCustId("cust-1");
+        assertDoesNotThrow(() -> customerDAO.updateCustomer(updated));
+        assertSame(updated, customerDAO.getCustomerById("cust-1"));
+
+        assertDoesNotThrow(() -> customerDAO.updateCustomer(null));
+        assertDoesNotThrow(() -> customerDAO.updateCustomer(new Customer()));
+        assertSame(updated, customerDAO.getCustomerById("cust-1"));
+    }
+
+    @Test
+    void planDaoInterfaceSupportsNullAndDoesNotForceImplementationBehavior() {
+        PlanDAO planDAO = new PlanDAO() {
+            @Override
+            public Plan AddPlan(String PlanId) {
+                return null;
+            }
+        };
+
+        assertNull(planDAO.AddPlan(null));
+        assertNull(planDAO.AddPlan("plan-1"));
+    }
+
+    private static void clearStaticMap(Class<?> type, String fieldName) throws Exception {
+        Field field = type.getDeclaredField(fieldName);
+        field.setAccessible(true);
+        @SuppressWarnings("unchecked")
+        Map<Object, Object> map = (Map<Object, Object>) field.get(null);
+        map.clear();
+    }
+}