diff --git a/src/TextMateSharp/Model/TMModel.cs b/src/TextMateSharp/Model/TMModel.cs
index fc14ced..5550583 100644
--- a/src/TextMateSharp/Model/TMModel.cs
+++ b/src/TextMateSharp/Model/TMModel.cs
@@ -430,7 +430,11 @@ public bool IsLineInvalid(int lineIndex)
 
         public void InvalidateLine(int lineIndex)
         {
-            this._lines.Get(lineIndex).IsInvalid = true;
+            var line = this._lines.Get(lineIndex);
+            if (line == null)
+                return;
+
+            line.IsInvalid = true;
 
             lock (_lock)
             {