Modify char in string

[Tim via Digitalmars-d-learn]

Hi everyone,

is there any chance to modify a char in a string like:
void main()
{
   string sMyText = "Replace the last char_";
   sMyText[$ - 1] = '.';
}

But when I execute the code above I'm always getting "cannot 
modify immutable expression at sMyText[__dollar -1LU]". I though 
D supported such modifications anytime. How can I do that now...?