diff --git a/CHANGELOG.md b/CHANGELOG.md
index eab0b7b..d792856 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,8 @@
 # Changelog
 
+## 0.13.3 | 2024-07-05
+* Fix deserialization for id in CustomFieldAttribute entity.
+
 ## 0.13.2 | 2024-06-26
 * Fix deserialization for sales_invoice in `CustomFieldSource` enum.
 
diff --git a/src/Moneybird.Net/Entities/Contacts/Contact.cs b/src/Moneybird.Net/Entities/Contacts/Contact.cs
index c5abf83..2980270 100644
--- a/src/Moneybird.Net/Entities/Contacts/Contact.cs
+++ b/src/Moneybird.Net/Entities/Contacts/Contact.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Text.Json.Serialization;
+using Moneybird.Net.Entities.CustomFields;
 using Moneybird.Net.Entities.Notes;
 using Moneybird.Net.Misc;
 
@@ -141,7 +142,7 @@ public class Contact : IMoneybirdEntity
         public List<Note> Notes { get; set; }
 
         [JsonPropertyName("custom_fields")]
-        public List<ContactCustomFieldsAttribute> CustomFields { get; set; }
+        public List<CustomFieldAttribute> CustomFields { get; set; }
 
         [JsonPropertyName("contact_people")]
         public List<ContactPerson> ContactPeople { get; set; }
diff --git a/src/Moneybird.Net/Entities/Contacts/ContactCustomFieldsAttribute.cs b/src/Moneybird.Net/Entities/Contacts/ContactCustomFieldsAttribute.cs
deleted file mode 100644
index 322ecf8..0000000
--- a/src/Moneybird.Net/Entities/Contacts/ContactCustomFieldsAttribute.cs
+++ /dev/null
@@ -1,13 +0,0 @@
-using System.Text.Json.Serialization;
-
-namespace Moneybird.Net.Entities.Contacts
-{
-    public class ContactCustomFieldsAttribute
-    {
-        [JsonPropertyName("id")]
-        public int Id { get; set; }
-        
-        [JsonPropertyName("value")]
-        public string Value { get; set; }
-    }
-}
\ No newline at end of file
diff --git a/src/Moneybird.Net/Entities/CustomFields/CustomFieldAttribute.cs b/src/Moneybird.Net/Entities/CustomFields/CustomFieldAttribute.cs
new file mode 100644
index 0000000..c91f94f
--- /dev/null
+++ b/src/Moneybird.Net/Entities/CustomFields/CustomFieldAttribute.cs
@@ -0,0 +1,17 @@
+using System.Text.Json.Serialization;
+
+namespace Moneybird.Net.Entities.CustomFields
+{
+    public class CustomFieldAttribute
+    {
+        [JsonPropertyName("id")]
+        public string Id { get; set; }
+        
+        [JsonPropertyName("name")]
+        [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+        public string Name { get; set; }
+
+        [JsonPropertyName("value")]
+        public string Value { get; set; }
+    }
+}
\ No newline at end of file
diff --git a/src/Moneybird.Net/Entities/ExternalSalesInvoices/ExternalSalesInvoice.cs b/src/Moneybird.Net/Entities/ExternalSalesInvoices/ExternalSalesInvoice.cs
index 1da276d..313a4b5 100644
--- a/src/Moneybird.Net/Entities/ExternalSalesInvoices/ExternalSalesInvoice.cs
+++ b/src/Moneybird.Net/Entities/ExternalSalesInvoices/ExternalSalesInvoice.cs
@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Text.Json.Serialization;
 using Moneybird.Net.Entities.Contacts;
+using Moneybird.Net.Entities.CustomFields;
 using Moneybird.Net.Entities.Payments;
 using Moneybird.Net.Entities.SalesInvoices;
 
@@ -101,7 +102,7 @@ public class ExternalSalesInvoice : IMoneybirdEntity
         public string PaymentUrl { get; set; }
 
         [JsonPropertyName("custom_fields")]
-        public List<ExternalSalesInvoiceCustomField> CustomFields { get; set; }
+        public List<CustomFieldAttribute> CustomFields { get; set; }
 
         [JsonPropertyName("tax_totals")]
         public List<SalesInvoiceTaxTotal> TaxTotals { get; set; }
diff --git a/src/Moneybird.Net/Entities/ExternalSalesInvoices/ExternalSalesInvoiceCustomField.cs b/src/Moneybird.Net/Entities/ExternalSalesInvoices/ExternalSalesInvoiceCustomField.cs
deleted file mode 100644
index b72c5b0..0000000
--- a/src/Moneybird.Net/Entities/ExternalSalesInvoices/ExternalSalesInvoiceCustomField.cs
+++ /dev/null
@@ -1,13 +0,0 @@
-using System.Text.Json.Serialization;
-
-namespace Moneybird.Net.Entities.ExternalSalesInvoices
-{
-    public class ExternalSalesInvoiceCustomField
-    {
-        [JsonPropertyName("id")]
-        public int Id { get; set; }
-
-        [JsonPropertyName("value")]
-        public string Value { get; set; }
-    }
-}
diff --git a/src/Moneybird.Net/Entities/SalesInvoices/SalesInvoice.cs b/src/Moneybird.Net/Entities/SalesInvoices/SalesInvoice.cs
index 4be3b60..330d3a8 100644
--- a/src/Moneybird.Net/Entities/SalesInvoices/SalesInvoice.cs
+++ b/src/Moneybird.Net/Entities/SalesInvoices/SalesInvoice.cs
@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Text.Json.Serialization;
 using Moneybird.Net.Entities.Contacts;
+using Moneybird.Net.Entities.CustomFields;
 using Moneybird.Net.Entities.Payments;
 
 namespace Moneybird.Net.Entities.SalesInvoices
@@ -165,7 +166,7 @@ public class SalesInvoice : IMoneybirdEntity
         public string PaymentUrl { get; set; }
 
         [JsonPropertyName("custom_fields")]
-        public List<SalesInvoiceCustomField> CustomFields { get; set; }
+        public List<CustomFieldAttribute> CustomFields { get; set; }
 
         [JsonPropertyName("tax_totals")]
         public List<SalesInvoiceTaxTotal> TaxTotals { get; set; }
diff --git a/src/Moneybird.Net/Entities/SalesInvoices/SalesInvoiceCustomField.cs b/src/Moneybird.Net/Entities/SalesInvoices/SalesInvoiceCustomField.cs
deleted file mode 100644
index a805dd3..0000000
--- a/src/Moneybird.Net/Entities/SalesInvoices/SalesInvoiceCustomField.cs
+++ /dev/null
@@ -1,13 +0,0 @@
-using System.Text.Json.Serialization;
-
-namespace Moneybird.Net.Entities.SalesInvoices
-{
-    public class SalesInvoiceCustomField
-    {
-        [JsonPropertyName("id")]
-        public int Id { get; set; }
-
-        [JsonPropertyName("value")]
-        public string Value { get; set; }
-    }
-}
diff --git a/src/Moneybird.Net/Models/Contacts/ContactCreateItem.cs b/src/Moneybird.Net/Models/Contacts/ContactCreateItem.cs
index bb4688b..2e96171 100644
--- a/src/Moneybird.Net/Models/Contacts/ContactCreateItem.cs
+++ b/src/Moneybird.Net/Models/Contacts/ContactCreateItem.cs
@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using System.Text.Json.Serialization;
 using Moneybird.Net.Entities.Contacts;
+using Moneybird.Net.Entities.CustomFields;
 using Moneybird.Net.Misc;
 
 namespace Moneybird.Net.Models.Contacts
@@ -101,7 +102,7 @@ public class ContactCreateItem
         public bool? DirectDebit { get; set; }
          
         [JsonPropertyName("custom_fields_attributes")]
-        public List<ContactCustomFieldsAttribute> CustomFieldsAttributes { get; set; }
+        public List<CustomFieldAttribute> CustomFieldsAttributes { get; set; }
          
         [JsonPropertyName("create_event")]
         public bool? CreateEvent { get; set; }
diff --git a/src/Moneybird.Net/Models/Contacts/ContactUpdateItem.cs b/src/Moneybird.Net/Models/Contacts/ContactUpdateItem.cs
index fa59462..540971a 100644
--- a/src/Moneybird.Net/Models/Contacts/ContactUpdateItem.cs
+++ b/src/Moneybird.Net/Models/Contacts/ContactUpdateItem.cs
@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using System.Text.Json.Serialization;
 using Moneybird.Net.Entities.Contacts;
+using Moneybird.Net.Entities.CustomFields;
 using Moneybird.Net.Misc;
 
 namespace Moneybird.Net.Models.Contacts
@@ -101,6 +102,6 @@ public class ContactUpdateItem
         public bool? DirectDebit { get; set; }
          
         [JsonPropertyName("custom_fields_attributes")]
-        public List<ContactCustomFieldsAttribute> CustomFieldsAttributes { get; set; }
+        public List<CustomFieldAttribute> CustomFieldsAttributes { get; set; }
     }
 }
\ No newline at end of file
diff --git a/src/Moneybird.Net/Models/SalesInvoices/SalesInvoiceCreate.cs b/src/Moneybird.Net/Models/SalesInvoices/SalesInvoiceCreate.cs
index 3cac54e..f243b71 100644
--- a/src/Moneybird.Net/Models/SalesInvoices/SalesInvoiceCreate.cs
+++ b/src/Moneybird.Net/Models/SalesInvoices/SalesInvoiceCreate.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Text.Json.Serialization;
+using Moneybird.Net.Entities.CustomFields;
 using Moneybird.Net.Entities.SalesInvoices;
 
 namespace Moneybird.Net.Models.SalesInvoices
@@ -70,6 +71,6 @@ public class SalesInvoiceCreate
         public IReadOnlyList<SalesInvoiceCreateDetail> DetailsAttributes { get; set; }
 
         [JsonPropertyName("custom_fields_attributes")]
-        public IReadOnlyList<SalesInvoiceCustomField> CustomFieldsAttributes { get; set; }
+        public IReadOnlyList<CustomFieldAttribute> CustomFieldsAttributes { get; set; }
     }
 }
diff --git a/src/Moneybird.Net/Models/SalesInvoices/SalesInvoiceUpdate.cs b/src/Moneybird.Net/Models/SalesInvoices/SalesInvoiceUpdate.cs
index 2929f1b..d796154 100644
--- a/src/Moneybird.Net/Models/SalesInvoices/SalesInvoiceUpdate.cs
+++ b/src/Moneybird.Net/Models/SalesInvoices/SalesInvoiceUpdate.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Text.Json.Serialization;
+using Moneybird.Net.Entities.CustomFields;
 using Moneybird.Net.Entities.SalesInvoices;
 
 namespace Moneybird.Net.Models.SalesInvoices
@@ -76,6 +77,6 @@ public class SalesInvoiceUpdate
         public IReadOnlyList<SalesInvoiceUpdateDetail> DetailsAttributes { get; set; }
 
         [JsonPropertyName("custom_fields_attributes")]
-        public IReadOnlyList<SalesInvoiceCustomField> CustomFieldsAttributes { get; set; }
+        public IReadOnlyList<CustomFieldAttribute> CustomFieldsAttributes { get; set; }
     }
 }
\ No newline at end of file
diff --git a/src/Moneybird.Net/Moneybird.Net.csproj b/src/Moneybird.Net/Moneybird.Net.csproj
index 7290580..62f43ef 100644
--- a/src/Moneybird.Net/Moneybird.Net.csproj
+++ b/src/Moneybird.Net/Moneybird.Net.csproj
@@ -3,7 +3,7 @@
         <TargetFramework>netstandard2.0</TargetFramework>
         <LangVersion>9</LangVersion>
         <PackageId>Moneybird.Net</PackageId>
-        <Version>0.13.1</Version>
+        <Version>0.13.3</Version>
         <Authors>Vincent Vrijburg</Authors>
         <Description>A wrapper for the Moneybird API.</Description>
         <Copyright>Copyright © Vincent Vrijburg 2021</Copyright>
@@ -12,7 +12,7 @@
         <PackageTags>dotnet dotnet-core dotnet-standard client wrapper moneybird</PackageTags>
         <PackageLicenseExpression>MIT</PackageLicenseExpression>
         <PackageRequireLicenseAcceptance>false</PackageRequireLicenseAcceptance>
-        <PackageVersion>0.13.1</PackageVersion>
+        <PackageVersion>0.13.3</PackageVersion>
     </PropertyGroup>
 
     <ItemGroup>
diff --git a/tests/Moneybird.Net.Tests/Endpoints/ContactEndpointTests.cs b/tests/Moneybird.Net.Tests/Endpoints/ContactEndpointTests.cs
index b47215f..81877f2 100644
--- a/tests/Moneybird.Net.Tests/Endpoints/ContactEndpointTests.cs
+++ b/tests/Moneybird.Net.Tests/Endpoints/ContactEndpointTests.cs
@@ -6,6 +6,7 @@
 using Moneybird.Net.Endpoints;
 using Moneybird.Net.Endpoints.Abstractions;
 using Moneybird.Net.Entities.Contacts;
+using Moneybird.Net.Entities.CustomFields;
 using Moneybird.Net.Entities.Notes;
 using Moneybird.Net.Http;
 using Moneybird.Net.Misc;
@@ -259,7 +260,14 @@ public async void CreateContactAsync_ByAccessToken_Returns_NewContact()
                     SiIdentifierTypeType = null,
                     CreateEvent = true,
                     DirectDebit = false,
-                    CustomFieldsAttributes = null
+                    CustomFieldsAttributes = new List<CustomFieldAttribute>
+                    {
+                        new ()
+                        {
+                            Id = "343816698630643114",
+                            Value = "Custom field test value"
+                        }
+                    }
                 }
             };
             
@@ -271,6 +279,7 @@ public async void CreateContactAsync_ByAccessToken_Returns_NewContact()
         
             var contact = JsonSerializer.Deserialize<Contact>(contactJson, _config.SerializerOptions);
             Assert.NotNull(contact);
+            Assert.NotNull(contact.CustomFields.First().Name);
 
             var actualContact = await _contactEndpoint.CreateAsync(AdministrationId, contactCreateOptions, AccessToken);
             Assert.NotNull(actualContact);
@@ -317,11 +326,11 @@ public async void UpdateContactAsync_ByAccessToken_Returns_UpdatedContact()
                     SiIdentifier = "",
                     SiIdentifierTypeType = null,
                     DirectDebit = false,
-                    CustomFieldsAttributes = new List<ContactCustomFieldsAttribute>
+                    CustomFieldsAttributes = new List<CustomFieldAttribute>
                     {
                         new ()
                         {
-                            Id = 1,
+                            Id = "1",
                             Value = "Custom contact field"
                         }
                     }
diff --git a/tests/Moneybird.Net.Tests/Endpoints/SalesInvoiceEndpointTests.cs b/tests/Moneybird.Net.Tests/Endpoints/SalesInvoiceEndpointTests.cs
index 6ecf0b1..7b3890f 100644
--- a/tests/Moneybird.Net.Tests/Endpoints/SalesInvoiceEndpointTests.cs
+++ b/tests/Moneybird.Net.Tests/Endpoints/SalesInvoiceEndpointTests.cs
@@ -5,6 +5,7 @@
 using System.Text.Json;
 using FluentAssertions;
 using Moneybird.Net.Endpoints;
+using Moneybird.Net.Entities.CustomFields;
 using Moneybird.Net.Entities.SalesInvoices;
 using Moneybird.Net.Http;
 using Moneybird.Net.Models.SalesInvoices;
@@ -155,9 +156,9 @@ public async void CreateSalesInvoiceAsync_ByAccessToken_Returns_NewSalesInvoice(
                 Discount = 0,
                 CustomFieldsAttributes = new []
                 {
-                    new SalesInvoiceCustomField
+                    new CustomFieldAttribute
                     {
-                        Id = 1,
+                        Id = "1",
                         Value = "Custom field value"
                     }
                 }
@@ -227,14 +228,14 @@ public async void UpdateSalesInvoiceAsync_ByAccessToken_Returns_UpdatedSalesInvo
                 },
                 CustomFieldsAttributes = new []
                 {
-                    new SalesInvoiceCustomField
+                    new CustomFieldAttribute()
                     {
-                        Id = 1,
+                        Id = "1",
                         Value = "Custom field value"
                     },
-                    new SalesInvoiceCustomField
+                    new CustomFieldAttribute
                     {
-                        Id = 2,
+                        Id = "2",
                         Value = "Second custom field value"
                     }
                 }
@@ -275,4 +276,4 @@ public async void AddSalesInvoiceAttachmentAsync_ByAccessToken_Returns()
         var file = new MemoryStream();
         await _salesInvoiceEndpoint.AddAttachmentAsync(AdministrationId, SalesInvoiceId, file, AccessToken, "testfile.pdf");
     }
-}
+}
\ No newline at end of file
diff --git a/tests/Moneybird.Net.Tests/Responses/Endpoints/Contacts/newContact.json b/tests/Moneybird.Net.Tests/Responses/Endpoints/Contacts/newContact.json
index 0c515e5..31d4fc0 100644
--- a/tests/Moneybird.Net.Tests/Responses/Endpoints/Contacts/newContact.json
+++ b/tests/Moneybird.Net.Tests/Responses/Endpoints/Contacts/newContact.json
@@ -43,7 +43,13 @@
   "version": 1640619493,
   "sales_invoices_url": "https://moneybird.com/322056713748350877/sales_invoices/6af2bbb86743f70b85c9371911a7ce22686d04069a3c858d6817c2cc3e5c8c4b/all",
   "notes": [],
-  "custom_fields": [],
+  "custom_fields": [
+    {
+      "id": "343816698630643114",
+      "name": "Test field",
+      "value": "Custom field test value"
+    }
+  ],
   "contact_people": [],
   "events": []
 }
\ No newline at end of file