diff --git a/Source/Plugins/BuilderModes/VisualModes/EffectSRB2ThingVertexSlope.cs b/Source/Plugins/BuilderModes/VisualModes/EffectSRB2ThingVertexSlope.cs
index 0d35e1d..ddd33e2 100644
--- a/Source/Plugins/BuilderModes/VisualModes/EffectSRB2ThingVertexSlope.cs
+++ b/Source/Plugins/BuilderModes/VisualModes/EffectSRB2ThingVertexSlope.cs
@@ -57,7 +57,7 @@ namespace CodeImp.DoomBuilder.BuilderModes
 				if (t.Parameter > 0)
 					position.z = t.GetFlagsValue();
 				else
-					position.z += t.Sector.FloorHeight;
+					position.z = (t.Sector != null) ? t.Sector.FloorHeight : 0;
                 verts[index] = position;
                 index++;
                 if (index > 2) break; //Only the first three vertices are used
diff --git a/Source/Plugins/BuilderModes/VisualModes/VisualMiddle3D.cs b/Source/Plugins/BuilderModes/VisualModes/VisualMiddle3D.cs
index 2163f6d..742ee4d 100644
--- a/Source/Plugins/BuilderModes/VisualModes/VisualMiddle3D.cs
+++ b/Source/Plugins/BuilderModes/VisualModes/VisualMiddle3D.cs
@@ -227,6 +227,9 @@ namespace CodeImp.DoomBuilder.BuilderModes
 			// Anything to see?
 			if(((cl - fl) > 0.01f) || ((cr - fr) > 0.01f))
 			{
+				if (extrafloor.Floor == null || extrafloor.Ceiling == null)
+					return false;
+
 				// Keep top and bottom planes for intersection testing
 				top = extrafloor.Floor.plane;
 				bottom = extrafloor.Ceiling.plane;