can not select field in a custom sharepoint designer workflow condition?

I have a custom workflow condition. It shows up in the designer, but when I selected a field, the value does not stay.

    <Condition Name="Detect list field value change"
       FunctionName ="ListItemPropertyChanged"
       ClassName="*****"
       AssemblyName ="*****"
       AppliesTo="list"
       Type="Custom"
       UsesCurrentItem="true">
      <RuleDesigner Sentence="Field: %1 in current list changed since previous version">
        <FieldBind Id="1" Field="left" DesignerType="FieldNames" Text="field"/>
      </RuleDesigner>
      <Parameters>
        <Parameter Name="left" Type="System.Object, mscorlib" Direction="In" />
      </Parameters>
    </Condition>

Could not find any document. This is similar to the one from the built-in.

Any help,

Thanks