diff --git a/src/Pipedrive.net/Converters/CustomFieldConverter.cs b/src/Pipedrive.net/Converters/CustomFieldConverter.cs
index 0edbce8d..07fefd0e 100644
--- a/src/Pipedrive.net/Converters/CustomFieldConverter.cs
+++ b/src/Pipedrive.net/Converters/CustomFieldConverter.cs
@@ -11,6 +11,17 @@ namespace Pipedrive.Internal
 {
     public class CustomFieldConverter : JsonConverter
     {
+        private readonly NullValueHandling? _nullValueHandling;
+
+        public CustomFieldConverter()
+        {
+        }
+
+        public CustomFieldConverter(NullValueHandling nullValueHandling)
+        {
+            _nullValueHandling = nullValueHandling;
+        }
+
         public override bool CanConvert(Type objectType)
         {
             return typeof(IEntityWithCustomFields).IsAssignableFrom(objectType);
@@ -178,9 +189,13 @@ public override void WriteJson(JsonWriter writer, object value, JsonSerializer s
                 if (property.Ignored) continue;
                 if (!ShouldSerialize(property, value)) continue;
 
-                var property_name = property.PropertyName;
                 var property_value = property.ValueProvider.GetValue(value);
 
+                if (property.NullValueHandling == NullValueHandling.Ignore && property_value == null) continue;
+                if (_nullValueHandling == NullValueHandling.Ignore && property_value == null) continue;
+
+                var property_name = property.PropertyName;
+
                 writer.WritePropertyName(property_name);
                 if (property.Converter != null && property.Converter.CanWrite)
                 {
diff --git a/src/Pipedrive.net/Models/Request/Deals/DealUpdate.cs b/src/Pipedrive.net/Models/Request/Deals/DealUpdate.cs
index aacee82f..ac421bac 100644
--- a/src/Pipedrive.net/Models/Request/Deals/DealUpdate.cs
+++ b/src/Pipedrive.net/Models/Request/Deals/DealUpdate.cs
@@ -6,7 +6,7 @@
 
 namespace Pipedrive
 {
-    [JsonConverter(typeof(CustomFieldConverter))]
+    [JsonConverter(typeof(CustomFieldConverter), NullValueHandling.Ignore)]
     public class DealUpdate : IEntityWithCustomFields
     {
         [JsonProperty("title")]
@@ -41,7 +41,7 @@ public class DealUpdate : IEntityWithCustomFields
         public string LostReason { get; set; }
 
         [JsonProperty("visible_to")]
-        public Visibility VisibleTo { get; set; }
+        public Visibility? VisibleTo { get; set; }
 
         [JsonProperty("add_time")]
         public DateTime? AddTime { get; set; }