diff --git a/Pilz.Net/Api/ApiResult.cs b/Pilz.Net/Api/ApiResult.cs
index 5f51a6a..1f22287 100644
--- a/Pilz.Net/Api/ApiResult.cs
+++ b/Pilz.Net/Api/ApiResult.cs
@@ -23,4 +23,6 @@ public record class ApiResult(
     public static ApiResult ServiceUnavailable() => new(HttpStatusCode.ServiceUnavailable);
 
     public static ApiResult UnavailableForLegalReasons() => new(HttpStatusCode.UnavailableForLegalReasons);
+
+    public static ApiResult InternalServerError() => new(HttpStatusCode.InternalServerError);
 }
\ No newline at end of file
diff --git a/Pilz.Net/Api/ApiServer.cs b/Pilz.Net/Api/ApiServer.cs
index d54577d..3c720f4 100644
--- a/Pilz.Net/Api/ApiServer.cs
+++ b/Pilz.Net/Api/ApiServer.cs
@@ -159,6 +159,8 @@ public class ApiServer(string apiUrl) : IApiServer
             isAuthenticated = CheckAuthentication(authKeyDecoded);
         else
             authKeyDecoded = null!;
+        if (attribute.RequiesAuth && !isAuthenticated)
+            return new(ApiResult.Unauthorized(), null);
 
         // Get required infos
         Log.Debug("Find other infos");
@@ -177,7 +179,7 @@ public class ApiServer(string apiUrl) : IApiServer
         Log.Debug("Invoke handler");
         var parameters = BuildParameters(handler, () => message, () => new(message, isAuthenticated, authKeyDecoded));
         if (handler.DynamicInvoke(parameters) is not ApiResult result)
-            return null;
+            return new(ApiResult.InternalServerError(), null);
 
         // Return result without message
         Log.Debug("Check message");
@@ -187,10 +189,10 @@ public class ApiServer(string apiUrl) : IApiServer
         // Serializer
         Log.Debug("Serialize message");
         if (serializer.Serialize(result.Message) is not string resultStr)
-            return null;
+            return new(ApiResult.InternalServerError(), null);
 
         // Return result with message
-        Log.Debug("Finish result");
+        Log.Debug("Complete result");
         return new(result, resultStr);
     }