what is different...?? (TBuf<64>...)

hello,
what is different??

When I have a local TBuf<64>  in my function, I can use all of it's methods like Append...
But when I define it in my header file(as a member variable)
I cann't use Zero or Append method:

in my header file I have defined it:

   TBuf<64> Name;

and in one of the functions I wrote:

   Name.Zero();

but I get this error:

error C2662: 'Zero' : cannot convert 'this' pointer from 'const class TBuf<64>' to 'class TDes16 &'


I don't undrestand...