setting strings in gdb

c++:

int main() 
    { 
    string a = "a"; 
    ... ... 
    }

when i debug in gdb:

(gdb) set var a = "ok"
Invalid cast

I run the program and pause at a break point after string a has been initialized. I'm trying to set its value, but it complains about invalid cast. What's the proper syntax for this?