diff --git a/Activout.RestClient/DomainExceptions/AbstractDomainExceptionMapper.cs b/Activout.RestClient/DomainExceptions/AbstractDomainExceptionMapper.cs
index 8bc7482..ddb4da6 100644
--- a/Activout.RestClient/DomainExceptions/AbstractDomainExceptionMapper.cs
+++ b/Activout.RestClient/DomainExceptions/AbstractDomainExceptionMapper.cs
@@ -1,4 +1,3 @@
-#nullable disable
 using System;
 using System.Net.Http;
 using System.Threading.Tasks;
@@ -7,14 +6,14 @@ namespace Activout.RestClient.DomainExceptions
 {
     public abstract class AbstractDomainExceptionMapper : IDomainExceptionMapper
     {
-        public virtual Task<Exception> CreateExceptionAsync(HttpResponseMessage httpResponseMessage, object data,
-            Exception innerException = null)
+        public virtual Task<Exception> CreateExceptionAsync(HttpResponseMessage httpResponseMessage, object? data,
+            Exception? innerException = null)
         {
             return Task.FromResult(CreateException(httpResponseMessage, data, innerException));
         }
 
-        protected virtual Exception CreateException(HttpResponseMessage httpResponseMessage, object data,
-            Exception innerException)
+        protected virtual Exception CreateException(HttpResponseMessage httpResponseMessage, object? data,
+            Exception? innerException)
         {
             throw new NotImplementedException();
         }
diff --git a/Activout.RestClient/DomainExceptions/DefaultDomainExceptionMapper.cs b/Activout.RestClient/DomainExceptions/DefaultDomainExceptionMapper.cs
index 03370a5..e988b9d 100644
--- a/Activout.RestClient/DomainExceptions/DefaultDomainExceptionMapper.cs
+++ b/Activout.RestClient/DomainExceptions/DefaultDomainExceptionMapper.cs
@@ -1,4 +1,3 @@
-#nullable disable
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
@@ -25,8 +24,8 @@ public DefaultDomainExceptionMapper(
             _httpErrorAttributes = httpErrorAttributes.ToList();
         }
 
-        protected override Exception CreateException(HttpResponseMessage httpResponseMessage, object data,
-            Exception innerException)
+        protected override Exception CreateException(HttpResponseMessage httpResponseMessage, object? data,
+            Exception? innerException)
         {
             var domainError = MapByDomainErrorAttribute(data)
                               ?? MapHttpStatusCodeByAttribute(httpResponseMessage)
@@ -35,15 +34,15 @@ protected override Exception CreateException(HttpResponseMessage httpResponseMes
 
             try
             {
-                return (Exception)Activator.CreateInstance(_domainExceptionType, domainError, innerException);
+                return (Exception)Activator.CreateInstance(_domainExceptionType, domainError, innerException)!;
             }
             catch (MissingMethodException)
             {
-                return (Exception)Activator.CreateInstance(_domainExceptionType, domainError);
+                return (Exception)Activator.CreateInstance(_domainExceptionType, domainError)!;
             }
         }
 
-        private static object MapByDomainErrorAttribute(object data)
+        private static object? MapByDomainErrorAttribute(object? data)
         {
             if (data == null)
             {
@@ -61,7 +60,7 @@ where a.ApiValue.Equals(value)
                     select a.DomainValue).FirstOrDefault();
         }
 
-        private object MapHttpStatusCodeByEnumName(HttpResponseMessage httpResponseMessage)
+        private object? MapHttpStatusCodeByEnumName(HttpResponseMessage httpResponseMessage)
         {
             try
             {
@@ -75,19 +74,19 @@ private object MapHttpStatusCodeByEnumName(HttpResponseMessage httpResponseMessa
             }
         }
 
-        private object MapHttpStatusCodeByAttribute(HttpResponseMessage httpResponseMessage)
+        private object? MapHttpStatusCodeByAttribute(HttpResponseMessage httpResponseMessage)
         {
             return GetDomainErrorValue(httpResponseMessage.StatusCode);
         }
 
-        private object GetDomainErrorValue(HttpStatusCode httpStatusCode)
+        private object? GetDomainErrorValue(HttpStatusCode httpStatusCode)
         {
             return _httpErrorAttributes
                 .FirstOrDefault(x => x.HttpStatusCode == httpStatusCode)
                 ?.DomainErrorValue;
         }
 
-        private object MapGenericClientOrServerError(HttpResponseMessage httpResponseMessage)
+        private object? MapGenericClientOrServerError(HttpResponseMessage httpResponseMessage)
         {
             var httpStatusCode = httpResponseMessage.StatusCode;
             return ((int)httpStatusCode > 500 ? GetDomainErrorValue((HttpStatusCode)500) : null)
diff --git a/Activout.RestClient/DomainExceptions/DefaultDomainExceptionMapperFactory.cs b/Activout.RestClient/DomainExceptions/DefaultDomainExceptionMapperFactory.cs
index 1ee547d..f845053 100644
--- a/Activout.RestClient/DomainExceptions/DefaultDomainExceptionMapperFactory.cs
+++ b/Activout.RestClient/DomainExceptions/DefaultDomainExceptionMapperFactory.cs
@@ -1,4 +1,3 @@
-#nullable disable
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
@@ -9,6 +8,8 @@ namespace Activout.RestClient.DomainExceptions
 {
     public class DefaultDomainExceptionMapperFactory : IDomainExceptionMapperFactory
     {
+        public static DefaultDomainExceptionMapperFactory Instance { get; } = new();
+
         public virtual IDomainExceptionMapper CreateDomainExceptionMapper(
             MethodInfo method,
             Type errorResponseType,
@@ -26,11 +27,17 @@ public virtual IDomainExceptionMapper CreateDomainExceptionMapper(
             return new DefaultDomainExceptionMapper(exceptionType, errorType, httpErrorAttributes);
         }
 
-        private static IEnumerable<DomainHttpErrorAttribute> GetAllDomainHttpErrorAttributes(MemberInfo method,
+        private static List<DomainHttpErrorAttribute> GetAllDomainHttpErrorAttributes(MemberInfo method,
             Type errorType)
         {
             var attributes = GetDomainHttpErrorAttributes(method).ToList();
-            attributes.AddRange(GetDomainHttpErrorAttributes(method.DeclaringType));
+
+            var declaringType = method.DeclaringType;
+            if (declaringType != null)
+            {
+                attributes.AddRange(GetDomainHttpErrorAttributes(declaringType));
+            }
+
             CheckDomainHttpErrorAttributes(errorType, attributes);
             return attributes;
         }
diff --git a/Activout.RestClient/DomainExceptions/DomainHttpErrorAttribute.cs b/Activout.RestClient/DomainExceptions/DomainHttpErrorAttribute.cs
index 34bf3ae..81b548f 100644
--- a/Activout.RestClient/DomainExceptions/DomainHttpErrorAttribute.cs
+++ b/Activout.RestClient/DomainExceptions/DomainHttpErrorAttribute.cs
@@ -1,11 +1,18 @@
 using System;
 using System.Net;
 
-namespace Activout.RestClient.DomainExceptions;
-
-[AttributeUsage(AttributeTargets.Interface | AttributeTargets.Method, AllowMultiple = true)]
-public class DomainHttpErrorAttribute(HttpStatusCode httpStatusCode, object domainErrorValue) : Attribute
+namespace Activout.RestClient.DomainExceptions
 {
-    public HttpStatusCode HttpStatusCode { get; } = httpStatusCode;
-    public object DomainErrorValue { get; } = domainErrorValue;
+    [AttributeUsage(AttributeTargets.Interface | AttributeTargets.Method, AllowMultiple = true)]
+    public class DomainHttpErrorAttribute : Attribute
+    {
+        public HttpStatusCode HttpStatusCode { get; }
+        public object DomainErrorValue { get; }
+
+        public DomainHttpErrorAttribute(HttpStatusCode httpStatusCode, object domainErrorValue)
+        {
+            HttpStatusCode = httpStatusCode;
+            DomainErrorValue = domainErrorValue;
+        }
+    }
 }
\ No newline at end of file
diff --git a/Activout.RestClient/DomainExceptions/IDomainExceptionMapper.cs b/Activout.RestClient/DomainExceptions/IDomainExceptionMapper.cs
index 1062cb8..e210afc 100644
--- a/Activout.RestClient/DomainExceptions/IDomainExceptionMapper.cs
+++ b/Activout.RestClient/DomainExceptions/IDomainExceptionMapper.cs
@@ -1,4 +1,3 @@
-#nullable disable
 using System;
 using System.Net.Http;
 using System.Threading.Tasks;
@@ -7,8 +6,8 @@ namespace Activout.RestClient.DomainExceptions
 {
     public interface IDomainExceptionMapper
     {
-        Task<Exception> CreateExceptionAsync(HttpResponseMessage httpResponseMessage, object data,
-            Exception innerException = null);
+        Task<Exception> CreateExceptionAsync(HttpResponseMessage httpResponseMessage, object? data,
+            Exception? innerException = null);
 
     }
 }
\ No newline at end of file