diff --git a/src/ACadSharp.Pdf/Core/IO/PdfPen.cs b/src/ACadSharp.Pdf/Core/IO/PdfPen.cs
index 0c72590..9a2767e 100644
--- a/src/ACadSharp.Pdf/Core/IO/PdfPen.cs
+++ b/src/ACadSharp.Pdf/Core/IO/PdfPen.cs
@@ -54,6 +54,8 @@ public void DrawEntity(Entity entity, Transform transform)
 				case IPolyline polyline:
 					this.drawPolyline(polyline, transform);
 					break;
+				case UnderlayEntity underlay:
+					throw new System.NotImplementedException();
 				case Viewport viewport:
 					this.drawViewport(viewport);
 					break;