diff --git a/src/main/java/com/example/bankapp/controller/BankController.java b/src/main/java/com/example/bankapp/controller/BankController.java
index 19fcded7..62724436 100644
--- a/src/main/java/com/example/bankapp/controller/BankController.java
+++ b/src/main/java/com/example/bankapp/controller/BankController.java
@@ -48,10 +48,18 @@ public String login() {
     }
 
     @PostMapping("/deposit")
-    public String deposit(@RequestParam BigDecimal amount) {
+    public String deposit(@RequestParam BigDecimal amount, Model model) {
         String username = SecurityContextHolder.getContext().getAuthentication().getName();
         Account account = accountService.findAccountByUsername(username);
-        accountService.deposit(account, amount);
+
+        try {
+            accountService.deposit(account, amount);
+        } catch (RuntimeException e) {
+            model.addAttribute("error", e.getMessage());
+            model.addAttribute("account", account);
+            return "dashboard";
+        }
+
         return "redirect:/dashboard";
     }
 
diff --git a/src/main/java/com/example/bankapp/service/AccountService.java b/src/main/java/com/example/bankapp/service/AccountService.java
index 5d7d90ec..806a7fb4 100644
--- a/src/main/java/com/example/bankapp/service/AccountService.java
+++ b/src/main/java/com/example/bankapp/service/AccountService.java
@@ -49,6 +49,7 @@ public Account registerAccount(String username, String password) {
 
 
     public void deposit(Account account, BigDecimal amount) {
+        validateAmount(amount);
         account.setBalance(account.getBalance().add(amount));
         accountRepository.save(account);
 
@@ -62,6 +63,7 @@ public void deposit(Account account, BigDecimal amount) {
     }
 
     public void withdraw(Account account, BigDecimal amount) {
+        validateAmount(amount);
         if (account.getBalance().compareTo(amount) < 0) {
             throw new RuntimeException("Insufficient funds");
         }
@@ -101,6 +103,7 @@ public Collection<? extends GrantedAuthority> authorities() {
     }
 
     public void transferAmount(Account fromAccount, String toUsername, BigDecimal amount) {
+        validateAmount(amount);
         if (fromAccount.getBalance().compareTo(amount) < 0) {
             throw new RuntimeException("Insufficient funds");
         }
@@ -134,4 +137,10 @@ public void transferAmount(Account fromAccount, String toUsername, BigDecimal am
         transactionRepository.save(creditTransaction);
     }
 
+    private static void validateAmount(BigDecimal amount) {
+        if (amount == null || amount.compareTo(BigDecimal.ZERO) <= 0) {
+            throw new RuntimeException("Amount must be greater than zero");
+        }
+    }
+
 }
diff --git a/src/test/java/com/example/bankapp/service/AccountServiceTest.java b/src/test/java/com/example/bankapp/service/AccountServiceTest.java
new file mode 100644
index 00000000..3429fb7a
--- /dev/null
+++ b/src/test/java/com/example/bankapp/service/AccountServiceTest.java
@@ -0,0 +1,117 @@
+package com.example.bankapp.service;
+
+import com.example.bankapp.model.Account;
+import com.example.bankapp.repository.AccountRepository;
+import com.example.bankapp.repository.TransactionRepository;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.security.crypto.password.PasswordEncoder;
+
+import java.math.BigDecimal;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoInteractions;
+
+@ExtendWith(MockitoExtension.class)
+class AccountServiceTest {
+
+    @Mock
+    AccountRepository accountRepository;
+
+    @Mock
+    TransactionRepository transactionRepository;
+
+    @Mock
+    PasswordEncoder passwordEncoder;
+
+    @InjectMocks
+    AccountService accountService;
+
+    @Test
+    void withdrawRejectsNegativeAmountWithoutChangingBalance() {
+        Account account = accountWithBalance("100");
+
+        assertThrows(RuntimeException.class,
+                () -> accountService.withdraw(account, new BigDecimal("-1000000")));
+
+        assertEquals(new BigDecimal("100"), account.getBalance());
+        verifyNoInteractions(accountRepository, transactionRepository);
+    }
+
+    @Test
+    void withdrawRejectsZeroAmount() {
+        Account account = accountWithBalance("100");
+
+        assertThrows(RuntimeException.class,
+                () -> accountService.withdraw(account, BigDecimal.ZERO));
+
+        assertEquals(new BigDecimal("100"), account.getBalance());
+        verifyNoInteractions(accountRepository, transactionRepository);
+    }
+
+    @Test
+    void withdrawRejectsNullAmount() {
+        Account account = accountWithBalance("100");
+
+        assertThrows(RuntimeException.class,
+                () -> accountService.withdraw(account, null));
+
+        assertEquals(new BigDecimal("100"), account.getBalance());
+        verifyNoInteractions(accountRepository, transactionRepository);
+    }
+
+    @Test
+    void withdrawUpdatesBalanceAndSavesTransaction() {
+        Account account = accountWithBalance("100");
+
+        accountService.withdraw(account, new BigDecimal("40"));
+
+        assertEquals(new BigDecimal("60"), account.getBalance());
+        verify(accountRepository).save(account);
+        verify(transactionRepository).save(org.mockito.ArgumentMatchers.any());
+    }
+
+    @Test
+    void depositRejectsNegativeAmountWithoutChangingBalance() {
+        Account account = accountWithBalance("100");
+
+        assertThrows(RuntimeException.class,
+                () -> accountService.deposit(account, new BigDecimal("-40")));
+
+        assertEquals(new BigDecimal("100"), account.getBalance());
+        verifyNoInteractions(accountRepository, transactionRepository);
+    }
+
+    @Test
+    void depositUpdatesBalanceAndSavesTransaction() {
+        Account account = accountWithBalance("100");
+
+        accountService.deposit(account, new BigDecimal("40"));
+
+        assertEquals(new BigDecimal("140"), account.getBalance());
+        verify(accountRepository).save(account);
+        verify(transactionRepository).save(org.mockito.ArgumentMatchers.any());
+    }
+
+    @Test
+    void transferRejectsNegativeAmountBeforeLookingUpRecipient() {
+        Account account = accountWithBalance("100");
+
+        assertThrows(RuntimeException.class,
+                () -> accountService.transferAmount(account, "recipient", new BigDecimal("-40")));
+
+        verifyNoInteractions(accountRepository, transactionRepository);
+    }
+
+    private Account accountWithBalance(String balance) {
+        Account account = new Account();
+        account.setUsername("account");
+        account.setBalance(new BigDecimal(balance));
+        return account;
+    }
+}