Adding a new Tab to an existing RCDC

I'm trying (and failing) to add a new tab to an existing RCDC.

I've read several posts on this and I cannot figure out what I'm missing.

I insert the following code in between 2 other groupings and every time I get the message "There is an error in the Group display configuration".

<my:Grouping my:Name="NewTab" my:Caption="NewTab"><my:Control my:Name="NewTabText" my:TypeName="UocLabel" my:Caption="NewTabText"><my:Properties><my:Property my:Name="Text" my:Value="!!!" /></my:Properties></my:Control></my:Grouping>

I've tried this code and mild variations of this code on several RCDCs and I always get a negative result. 

Any help, thoughts appreciated.  If I do another iisreset today I might explode.

Thanks