diff --git a/src/RobinTTY.NordigenApiClient.Tests/LiveApi/CredentialTests.cs b/src/RobinTTY.NordigenApiClient.Tests/LiveApi/CredentialTests.cs
index 228cd51..de59c57 100644
--- a/src/RobinTTY.NordigenApiClient.Tests/LiveApi/CredentialTests.cs
+++ b/src/RobinTTY.NordigenApiClient.Tests/LiveApi/CredentialTests.cs
@@ -1,4 +1,5 @@
 using RobinTTY.NordigenApiClient.Models;
+using RobinTTY.NordigenApiClient.Models.Jwt;
 using RobinTTY.NordigenApiClient.Tests.Shared;
 
 namespace RobinTTY.NordigenApiClient.Tests.LiveApi;
@@ -32,7 +33,7 @@ public async Task CheckValidTokensAfterRequest()
     }
 
     /// <summary>
-    /// Tests the failure of authentication when trying to execute a request.
+    /// Tests the failure of authentication due to invalid credentials when trying to execute a request.
     /// </summary>
     [Test]
     public async Task ExecuteRequestWithInvalidCredentials()
@@ -42,8 +43,8 @@ public async Task ExecuteRequestWithInvalidCredentials()
             "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef");
         var apiClient = new NordigenClient(httpClient, invalidCredentials);
 
-        // Returns BasicError
         var agreementsResponse = await apiClient.TokenEndpoint.GetTokenPair();
+
         Assert.Multiple(() =>
         {
             AssertionHelpers.AssertNordigenApiResponseIsUnsuccessful(agreementsResponse, HttpStatusCode.Unauthorized);
@@ -52,6 +53,28 @@ public async Task ExecuteRequestWithInvalidCredentials()
         });
     }
 
+    /// <summary>
+    /// Tests the failure of authentication due to an invalid token when trying to execute a request.
+    /// </summary>
+    [Test]
+    public async Task ExecuteRequestWithUnauthorizedToken()
+    {
+        using var httpClient = new HttpClient();
+        var invalidCredentials = new NordigenClientCredentials("01234567-89ab-cdef-0123-456789abcdef",
+            "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef");
+        var token = new JsonWebTokenPair(TestHelpers.Secrets[14], TestHelpers.Secrets[14]);
+        var apiClient = new NordigenClient(httpClient, invalidCredentials, token);
+
+        var response = await apiClient.InstitutionsEndpoint.GetInstitutions();
+
+        Assert.Multiple(() =>
+        {
+            AssertionHelpers.AssertNordigenApiResponseIsUnsuccessful(response, HttpStatusCode.Unauthorized);
+            AssertionHelpers.AssertBasicResponseMatchesExpectations(response.Error, "Invalid token",
+                "Token is invalid or expired");
+        });
+    }
+
     /// <summary>
     /// Tries to execute a request using credentials that haven't whitelisted the used IP. This should cause an error.
     /// </summary>
diff --git a/src/RobinTTY.NordigenApiClient.Tests/LiveApi/Endpoints/AccountsEndpointTests.cs b/src/RobinTTY.NordigenApiClient.Tests/LiveApi/Endpoints/AccountsEndpointTests.cs
index 11e710d..273b4ac 100644
--- a/src/RobinTTY.NordigenApiClient.Tests/LiveApi/Endpoints/AccountsEndpointTests.cs
+++ b/src/RobinTTY.NordigenApiClient.Tests/LiveApi/Endpoints/AccountsEndpointTests.cs
@@ -6,12 +6,15 @@ namespace RobinTTY.NordigenApiClient.Tests.LiveApi.Endpoints;
 public class AccountsEndpointTests
 {
     private Guid _accountId;
+    private Guid _nonExistingAccountId;
+    private const string InvalidGuid = "abcdefg";
     private NordigenClient _apiClient = null!;
 
     [OneTimeSetUp]
     public void Setup()
     {
         _accountId = Guid.Parse(TestHelpers.Secrets[9]);
+        _nonExistingAccountId = Guid.Parse("f1d53c46-260d-4556-82df-4e5fed58e37c");
         _apiClient = TestHelpers.GetConfiguredClient();
     }
 
@@ -121,6 +124,21 @@ await _apiClient.AccountsEndpoint.GetTransactions(_accountId, startDate,
     #endregion
 
     #region RequestsWithErrors
+    
+    /// <summary>
+    /// Tests the retrieval of an account that does not exist. This should return an error.
+    /// </summary>
+    [Test]
+    public async Task GetAccountWithInvalidGuid()
+    {
+        var accountResponse = await _apiClient.AccountsEndpoint.GetAccount(InvalidGuid);
+        
+        Assert.Multiple(() =>
+        {
+            AssertionHelpers.AssertNordigenApiResponseIsUnsuccessful(accountResponse, HttpStatusCode.BadRequest);
+            AssertionHelpers.AssertBasicResponseMatchesExpectations(accountResponse.Error, "Invalid Account ID", $"{InvalidGuid} is not a valid Account UUID. ");
+        });
+    }
 
     /// <summary>
     /// Tests the retrieval of an account that does not exist. This should return an error.
@@ -128,7 +146,7 @@ await _apiClient.AccountsEndpoint.GetTransactions(_accountId, startDate,
     [Test]
     public async Task GetAccountThatDoesNotExist()
     {
-        var accountResponse = await _apiClient.AccountsEndpoint.GetAccount("f1d53c46-260d-4556-82df-4e5fed58e37c");
+        var accountResponse = await _apiClient.AccountsEndpoint.GetAccount(_nonExistingAccountId);
         
         Assert.Multiple(() =>
         {
@@ -137,6 +155,21 @@ public async Task GetAccountThatDoesNotExist()
         });
     }
 
+    /// <summary>
+    /// Tests the retrieval of balances of an account that does not exist. This should return an error.
+    /// </summary>
+    [Test]
+    public async Task GetBalancesForAccountThatDoesNotExist()
+    {
+        var balancesResponse = await _apiClient.AccountsEndpoint.GetBalances(_nonExistingAccountId);
+        
+        Assert.Multiple(() =>
+        {
+            AssertionHelpers.AssertNordigenApiResponseIsUnsuccessful(balancesResponse, HttpStatusCode.NotFound);
+            AssertionHelpers.AssertBasicResponseMatchesExpectations(balancesResponse.Error, $"Account ID {_nonExistingAccountId} not found", "Please check whether you specified a valid Account ID");
+        });
+    }
+
     /// <summary>
     /// Tests the retrieval of transactions within a specific time frame in the future. This should return an error.
     /// </summary>
diff --git a/src/RobinTTY.NordigenApiClient.Tests/Mocks/Endpoints/AccountsEndpointTests.cs b/src/RobinTTY.NordigenApiClient.Tests/Mocks/Endpoints/AccountsEndpointTests.cs
index 273c3d8..685edc2 100644
--- a/src/RobinTTY.NordigenApiClient.Tests/Mocks/Endpoints/AccountsEndpointTests.cs
+++ b/src/RobinTTY.NordigenApiClient.Tests/Mocks/Endpoints/AccountsEndpointTests.cs
@@ -6,6 +6,8 @@ namespace RobinTTY.NordigenApiClient.Tests.Mocks.Endpoints;
 
 public class AccountsEndpointTests
 {
+    private const string InvalidGuid = "abcdefg";
+
     /// <summary>
     /// Tests the retrieval of an account.
     /// </summary>
@@ -126,6 +128,23 @@ await apiClient.AccountsEndpoint.GetTransactions(A.Dummy<Guid>(), startDate,
         AssertionHelpers.AssertNordigenApiResponseIsSuccessful(balancesResponse, HttpStatusCode.OK);
         Assert.That(balancesResponse.Result!.BookedTransactions, Has.Count.EqualTo(2));
     }
+    
+    /// <summary>
+    /// Tests the retrieval of an account that does not exist. This should return an error.
+    /// </summary>
+    [Test]
+    public async Task GetAccountWithInvalidGuid()
+    {
+        var apiClient = TestHelpers.GetMockClient(TestHelpers.MockData.AccountsEndpointMockData.GetAccountWithInvalidGuid, HttpStatusCode.BadRequest);
+        
+        var accountResponse = await apiClient.AccountsEndpoint.GetAccount(InvalidGuid);
+        
+        Assert.Multiple(() =>
+        {
+            AssertionHelpers.AssertNordigenApiResponseIsUnsuccessful(accountResponse, HttpStatusCode.BadRequest);
+            AssertionHelpers.AssertBasicResponseMatchesExpectations(accountResponse.Error, "Invalid Account ID", $"{InvalidGuid} is not a valid Account UUID. ");
+        });
+    }
 
     /// <summary>
     /// Tests the retrieval of an account that does not exist. This should return an error.
@@ -143,6 +162,24 @@ public async Task GetAccountThatDoesNotExist()
             AssertionHelpers.AssertBasicResponseMatchesExpectations(accountResponse.Error, "Not found.", "Not found.");
         });
     }
+    
+    /// <summary>
+    /// Tests the retrieval of balances of an account that does not exist. This should return an error.
+    /// </summary>
+    [Test]
+    public async Task GetBalancesForAccountThatDoesNotExist()
+    {
+        var apiClient = TestHelpers.GetMockClient(TestHelpers.MockData.AccountsEndpointMockData.GetBalancesForAccountThatDoesNotExist, HttpStatusCode.NotFound);
+        
+        var nonExistingAccountId = Guid.Parse("f1d53c46-260d-4556-82df-4e5fed58e37c");
+        var balancesResponse = await apiClient.AccountsEndpoint.GetBalances(A.Dummy<Guid>());
+        
+        Assert.Multiple(() =>
+        {
+            AssertionHelpers.AssertNordigenApiResponseIsUnsuccessful(balancesResponse, HttpStatusCode.NotFound);
+            AssertionHelpers.AssertBasicResponseMatchesExpectations(balancesResponse.Error, $"Account ID {nonExistingAccountId} not found", "Please check whether you specified a valid Account ID");
+        });
+    }
 
     /// <summary>
     /// Tests the retrieval of transactions within a specific time frame in the future. This should return an error.
diff --git a/src/RobinTTY.NordigenApiClient.Tests/Mocks/Responses/MockResponsesModel.cs b/src/RobinTTY.NordigenApiClient.Tests/Mocks/Responses/MockResponsesModel.cs
index 2ecde16..f1f5384 100644
--- a/src/RobinTTY.NordigenApiClient.Tests/Mocks/Responses/MockResponsesModel.cs
+++ b/src/RobinTTY.NordigenApiClient.Tests/Mocks/Responses/MockResponsesModel.cs
@@ -27,8 +27,9 @@ internal class AccountsEndpointMockData(
     AccountTransactionsWrapper getTransactions,
     AccountTransactionsWrapper getTransactionRange,
     AccountsError getTransactionRangeInFuture,
-    AccountsError getAccountThatDoesNotExist
-    )
+    AccountsError getAccountWithInvalidGuid,
+    AccountsError getAccountThatDoesNotExist,
+    AccountsError getBalancesForAccountThatDoesNotExist)
 {
     public BankAccount GetAccount { get; set; } = getAccount;
     public BalanceJsonWrapper GetBalances { get; set; } = getBalances;
@@ -36,7 +37,9 @@ AccountsError getAccountThatDoesNotExist
     public AccountTransactionsWrapper GetTransactions { get; set; } = getTransactions;
     public AccountTransactionsWrapper GetTransactionRange { get; set; } = getTransactionRange;
     public AccountsError GetTransactionRangeInFuture { get; set; } = getTransactionRangeInFuture;
+    public AccountsError GetAccountWithInvalidGuid { get; set; } = getAccountWithInvalidGuid;
     public AccountsError GetAccountThatDoesNotExist { get; set; } = getAccountThatDoesNotExist;
+    public AccountsError GetBalancesForAccountThatDoesNotExist { get; set; } = getBalancesForAccountThatDoesNotExist;
 }
 
 internal class AgreementsEndpointMockData(
diff --git a/src/RobinTTY.NordigenApiClient.Tests/Mocks/Responses/responses.json b/src/RobinTTY.NordigenApiClient.Tests/Mocks/Responses/responses.json
index ea2a600..1a00ef2 100644
--- a/src/RobinTTY.NordigenApiClient.Tests/Mocks/Responses/responses.json
+++ b/src/RobinTTY.NordigenApiClient.Tests/Mocks/Responses/responses.json
@@ -124,11 +124,21 @@
         ]
       }
     },
+    "GetAccountWithInvalidGuid": {
+      "summary": "Invalid Account ID",
+      "detail": "abcdefg is not a valid Account UUID. ",
+      "status_code": 400
+    },
     "GetAccountThatDoesNotExist": {
       "detail": "Not found.",
       "summary": "Not found.",
       "status_code": 404
     },
+    "GetBalancesForAccountThatDoesNotExist": {
+      "summary": "Account ID f1d53c46-260d-4556-82df-4e5fed58e37c not found",
+      "detail": "Please check whether you specified a valid Account ID",
+      "status_code": 404
+    },
     "GetTransactionRangeInFuture": {
       "date_from": {
         "summary": "Date can't be in future",