diff --git a/Xbim.Tessellator/Tessellation/Sweep.cs b/Xbim.Tessellator/Tessellation/Sweep.cs
index 4c74de43b..07b542608 100644
--- a/Xbim.Tessellator/Tessellation/Sweep.cs
+++ b/Xbim.Tessellator/Tessellation/Sweep.cs
@@ -372,6 +372,10 @@ private void GetIntersectData(MeshUtils.Vertex isect, MeshUtils.Vertex orgUp, Me
                     new[] { w0, w1, w2, w3 }
                 );
             }
+            else
+            {
+                isect._data = -1;
+            }
         }
 
         /// <summary>