Can not specify the value "true" for attribute "syncnamechanges"

Hi,

I am working on customization of the Test Case work item, I have added new field as below :

  <FIELD name="Designed By" refname="MyComp.DesignedBy"type="String"syncnamechanges="true" reportable="dimension">     

<ALLOWEXISTINGVALUE />
        <VALIDUSER />

</FIELD>

I am facing below validation error:

TF212017: Work item tracking schema validation warning: TF248017: You cannot specify the "renamesafe" attribute for field 'MyComp.DesignedBy'. You can only specify the "syncnamechanges" attribute for a field whose data type is "String".

Please help me fixing this issue.