diff --git a/Source/Plugins/BuilderModes/VisualModes/VisualCeiling.cs b/Source/Plugins/BuilderModes/VisualModes/VisualCeiling.cs
index 730dc2a..7995239 100644
--- a/Source/Plugins/BuilderModes/VisualModes/VisualCeiling.cs
+++ b/Source/Plugins/BuilderModes/VisualModes/VisualCeiling.cs
@@ -241,7 +241,7 @@ namespace CodeImp.DoomBuilder.BuilderModes
 			{
 				Linedef alignment = null;
 				foreach (Sidedef side in s.Sidedefs)
-					if (side.Line.IsFlatAlignment && side.Line.Tag == 0 && side.Sector == s) alignment = side.Line;
+					if (side.Line.IsFlatAlignment && side.Line.Tag == 0 && side.Line.Front.Sector == s) alignment = side.Line;
 				if (alignment == null)
 				{
 					mode.SetActionResult("Sector has no flat alignment line!");
diff --git a/Source/Plugins/BuilderModes/VisualModes/VisualFloor.cs b/Source/Plugins/BuilderModes/VisualModes/VisualFloor.cs
index a6662c7..ffc0200 100644
--- a/Source/Plugins/BuilderModes/VisualModes/VisualFloor.cs
+++ b/Source/Plugins/BuilderModes/VisualModes/VisualFloor.cs
@@ -229,7 +229,7 @@ namespace CodeImp.DoomBuilder.BuilderModes
 			{
 				Linedef alignment = null;
 				foreach (Sidedef side in s.Sidedefs)
-					if (side.Line.IsFlatAlignment && side.Line.Tag == 0 && side.Sector == s) alignment = side.Line;
+					if (side.Line.IsFlatAlignment && side.Line.Tag == 0 && side.Line.Front.Sector == s) alignment = side.Line;
 				if (alignment == null)
 				{
 					mode.SetActionResult("Sector has no flat alignment line!");