C# -DirectX kode fejler

Hvad er problemet i denne kode?
            VertexBuffer vb = (VertexBuffer)sender;
            GraphicsStream stm = vb.Lock(0, 0, 0);
            CustomVertex.TransformedColored[] verts = new CustomVertex.TransformedColored[4];

            verts[0].X=100;verts[0].Y=50;verts[0].Z=0.5f; verts[0].Rhw=1; verts[0].Color = System.Drawing.Color.Beige.ToArgb();
            verts[1].X=100;verts[1].Y=100;verts[1].Z=0.5f; verts[1].Rhw=1; verts[1].Color = System.Drawing.Color.Brown.ToArgb();
            verts[2].X=50;verts[2].Y=250;verts[2].Z=0.5f; verts[2].Rhw=1; verts[2].Color = System.Drawing.Color.LightPink.ToArgb();
          verts[3].X = 50; verts[3].Y = 300; verts[3].Z = 0.5f; verts[3].Rhw = 1;verts[3].Color = System.Drawing.Color.Black.ToArgb();
            stm.Write(verts);
            vb.Unlock();

får denne fejl ved stm.Write(verts);
Operation is not valid due to the current state of the object.