error when saving custom user field in a custom Page Layout

I have created a custom user field as follows

<Field
  ID="{9B49F1A5-7FFB-400B-9B0E-0FC4D6C1417C}"
  Name="AMtalatFieldEditors"
  DisplayName="My Editors"
  Type="UserMulti"
  Mult="TRUE"
  Required="TRUE"
  ShowField="FirstName" 
  StaticName="AMtalatFieldEditors"
  SourceID="http://schemas.microsoft.com/sharepoint/v3"
  Customization=""
  UserSelectionMode="PeopleOnly"
  UserSelectionScope="0"
  Group="MyGroup"
  ShowInNewForm="TRUE">
</Field>

added it to my content type

  <FieldRef ID="{9B49F1A5-7FFB-400B-9B0E-0FC4D6C1417C}" Name="AMtalatFieldEditors" DisplayName="My Editors" Required="TRUE" Customization="" ShowInNewForm="TRUE"/>

add it to the page layout

 <div id="editorsDiv">
            <SharePointWebControls:UserField ID="AMtalatFieldEditors" FieldName="AMtalatFieldEditors" runat="server" />
        </div>

I edit a page and change the page layout to the mine and fill out the user field and click on Save. But the value of user field is disappeared and when I click on Check in, it says:
Error: you have to fill out the required property and when I click on “OK” it redirects me to the page to fill the properties. it seems I never can check in my pages.