delphi 2009 string vs opengl shader commands

[noeska]

System: WindowsXP
Compiler/IDE: Delphi2009
Libraries/API: DglOpengl

I am puzzled again with the new delphi string and the likes :-(
On calling a shader with parameters the actual parameter is never set.

procedure TGLSLProgram.SetF(aname: string; arg: GLFloat);
begin
 glUniform1fARB(glGetUniformLocationARB(FProgramObject, @aname ), arg);
end;

But when is use the variable name directly here:

procedure TGLSLProgram.SetF(aname: string; arg: GLFloat);
begin
 glUniform1fARB(glGetUniformLocationARB(FProgramObject, 'test' ), arg);
end;

Then it does works.

But for now i am unable to provide a working type. String / AnsiString / PChar / PAnsiChar but none seem to work :-(