diff --git a/Terminal/TerminalBase.cs b/Terminal/TerminalBase.cs
index 0921853..a484b71 100644
--- a/Terminal/TerminalBase.cs
+++ b/Terminal/TerminalBase.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics.Contracts;
 using System.IO;
 using System.Linq;
 using System.Text;
@@ -189,6 +190,10 @@ void advanceCursorRow()
 
 		public void MoveLines(int index, int newIndex, int count)
 		{
+			Contract.Requires(index >= 0 && index < Size.Row);
+			Contract.Requires(newIndex >= 0 && newIndex < Size.Row);
+			Contract.Requires(index + count < Size.Row && newIndex + count < Size.Row);
+
 			int addIndex;
 			if (newIndex > index)
 				addIndex = index;
@@ -206,16 +211,22 @@ public void MoveLines(int index, int newIndex, int count)
 
 		public void EraseCharacters(int length, bool advanceCursor = true)
 		{
+			Contract.Requires(length >= 0 && CursorPos.Col + length < Size.Col);
+
 			SetCharacters(new string(' ', length), new TerminalFont() { Hidden = true }, advanceCursor);
 		}
 
 		public void DeleteCharacters(int length)
 		{
+			Contract.Requires(length >= 0 && CursorPos.Col + length < Size.Col);
+
 			lines[CursorPos.Row].DeleteCharacters(CursorPos.Col, length);
 		}
 
 		public void SetCharacters(string text, TerminalFont font, bool advanceCursor = true)
 		{
+			Contract.Requires(text != null);
+
 			bool ignoreNextLine = false;
 			int textIndex = 0;
 			while (textIndex < text.Length)
diff --git a/Terminal/XtermTerminal.cs b/Terminal/XtermTerminal.cs
index 6887cde..f977844 100644
--- a/Terminal/XtermTerminal.cs
+++ b/Terminal/XtermTerminal.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.Contracts;
 using System.IO;
 using System.Linq;
 using System.Text;
@@ -257,13 +258,10 @@ public TerminalFont DefaultFont
 
 		void setPrivateMode(XtermDecMode mode, bool value)
 		{
-			if (Enum.IsDefined(typeof(XtermDecMode), mode))
-			{
-				privateModes[(XtermDecMode) mode] = value;
+			privateModes[mode] = value;
 
-				if (PrivateModeChanged != null)
-					PrivateModeChanged(this, new PrivateModeChangedEventArgs(mode, value));
-			}
+			if (PrivateModeChanged != null)
+				PrivateModeChanged(this, new PrivateModeChangedEventArgs(mode, value));
 		}
 
 		public event EventHandler<TitleChangeEventArgs> TitleChanged;
@@ -272,6 +270,8 @@ void setPrivateMode(XtermDecMode mode, bool value)
 		public XtermTerminal(IStreamNotifier streamNotifier)
 			: base(streamNotifier)
 		{
+			Contract.Requires(streamNotifier != null);
+
 			foreach (var key in Enum.GetValues(typeof(XtermDecMode)).Cast<XtermDecMode>())
 				privateModes.Add(key, false);
 			privateModes[XtermDecMode.BlinkCursor] = true;