CSP - using cspbind dynamically

Hi

I'm trying to create a form on a csp page, with input elements binding to data using the cspbind attribute.

All is working fine when i use string literal as in

But trying  to set the attribute to runtime expression  doesn't work for me, i.e:

 The same rt expression produce the desired string when used in other elements, i.e:

-> will produce 'UserName' on page.
 

Are there any known limitations for this attribute?   

I'm using version:

Cache for Windows (x86-64) 2015.2 (Build 664

Thanks