Required argument in a XAML activity

Hi all!

I'm working with Windows Workflow Foundation 4. I created an activity with the VS 2010 designer, so it creates a XAML file for it. The contents is as follows:

<Activity mc:Ignorable="sap" x:Class="WorkflowConsoleApplication2.Workflow1" xmlns="http://schemas.microsoft.com/netfx/2009/xaml/activities" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:mv="clr-namespace:Microsoft.VisualBasic;assembly=System" xmlns:mva="clr-namespace:Microsoft.VisualBasic.Activities;assembly=System.Activities" xmlns:s="clr-namespace:System;assembly=mscorlib" xmlns:s1="clr-namespace:System;assembly=System" xmlns:s2="clr-namespace:System;assembly=System.Xml" xmlns:s3="clr-namespace:System;assembly=System.Core" xmlns:sad="clr-namespace:System.Activities.Debugger;assembly=System.Activities" xmlns:sap="http://schemas.microsoft.com/netfx/2009/xaml/activities/presentation" xmlns:scg="clr-namespace:System.Collections.Generic;assembly=System" xmlns:scg1="clr-namespace:System.Collections.Generic;assembly=System.ServiceModel" xmlns:scg2="clr-namespace:System.Collections.Generic;assembly=System.Core" xmlns:scg3="clr-namespace:System.Collections.Generic;assembly=mscorlib" xmlns:sd="clr-namespace:System.Data;assembly=System.Data" xmlns:sl="clr-namespace:System.Linq;assembly=System.Core" xmlns:st="clr-namespace:System.Text;assembly=mscorlib" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <x:Members>
    <x:Property Name="argument1" Type="InArgument(x:String)">
      <x:Property.Attributes>
        <RequiredArgumentAttribute />
      </x:Property.Attributes>
    </x:Property>
  </x:Members>
  <sap:VirtualizedContainerService.HintSize>240,240</sap:VirtualizedContainerService.HintSize>
  <mva:VisualBasic.Settings>Assembly references and imported namespaces for internal implementation</mva:VisualBasic.Settings>
</Activity>

Notice the required argument I created. R# 5.1.1753.4 marks the lines with x:Property.Attributes with an error, saying "Cannot resolve symbol 'Attributes'".

Do I miss something?

Thanks in advance,

Malte