I want draw some text string in opengl (C#, Tao) by using following code

Visual studio version 2010
Tao framework 2 (latest from source)
I have referenced the dll corrctly Tao.OpenGL, Tao.FreeGlut, Tao.Flatform.Windows

The problem is when it comes to 3rd line (Glut.glutBitmapCharacter(Glut.GLUT_BITMAP_HELVETI CA_18, c)) in the method, it crashing, why is that? How to tackle the problem?

private void PrintText(float x, float y, string text) {
Gl.glRasterPos2f(x, y);
foreach(char c in text) {
Glut.glutBitmapCharacter(Glut.GLUT_BITMAP_HELVETIC A_18, c);
}
}