diff --git a/Source/Core/Map/MapSet.cs b/Source/Core/Map/MapSet.cs
index bf891eb..89be2b6 100644
--- a/Source/Core/Map/MapSet.cs
+++ b/Source/Core/Map/MapSet.cs
@@ -3387,6 +3387,9 @@ namespace CodeImp.DoomBuilder.Map
 		/// <summary>This finds the line closest to the specified position.</summary>
 		public static Linedef NearestLinedefRange(BlockMap<BlockEntry> selectionmap, Vector2D pos, float maxrange) //mxd
 		{
+			if (selectionmap == null)
+				return null;
+
 			Linedef closest = null;
 			float distance = float.MaxValue;
 			float maxrangesq = maxrange * maxrange;