diff --git a/SuperVM.Assembler/Assembler.cs b/SuperVM.Assembler/Assembler.cs
index 7848148..04db48e 100644
--- a/SuperVM.Assembler/Assembler.cs
+++ b/SuperVM.Assembler/Assembler.cs
@@ -204,6 +204,9 @@ namespace SuperVM.Assembler
 				foreach (var patch in patches)
 				{
 					var target = patch.Value;
+					if (labels.ContainsKey(target) == false)
+						throw new InvalidOperationException($"Missing label: {target}");
+
 					var position = labels[target];
 					code[patch.Key] =
 						(code[patch.Key] & 0xFFFFFFFF) |
diff --git a/SuperVM.VisualDebugger/MainWindow.xaml b/SuperVM.VisualDebugger/MainWindow.xaml
index 79c350a..7500d78 100644
--- a/SuperVM.VisualDebugger/MainWindow.xaml
+++ b/SuperVM.VisualDebugger/MainWindow.xaml
@@ -6,7 +6,8 @@
         xmlns:local="clr-namespace:SuperVM.VisualDebugger"
 		xmlns:avalon="clr-namespace:ICSharpCode.AvalonEdit;assembly=ICSharpCode.AvalonEdit"
         mc:Ignorable="d"
-        Title="MainWindow" Height="350" Width="525">
+		WindowStartupLocation="CenterScreen"
+        Title="MainWindow" Height="768" Width="1024">
 	<Window.DataContext>
 		<local:VirtualMachineModel />
 	</Window.DataContext>
@@ -125,7 +126,7 @@
 						AcceptsReturn="True"
 						FontFamily="Consolas"
 						VerticalContentAlignment="Top" 
-						Text="{Binding Path=Output}"/>
+						Text="{Binding Path=Output, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
 				</TabItem>
 				<TabItem Header="Memory" Grid.IsSharedSizeScope="True">
 					<DockPanel>
@@ -164,6 +165,7 @@
 						FontFamily="Consolas"
 						Background="#222"
 						Foreground="#DDD"
+						FontSize="16"
 						Code="{Binding Path=Source, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
 				</TabItem>
 				<TabItem Header="Code">
diff --git a/SuperVM.VisualDebugger/VirtualMachineModel.cs b/SuperVM.VisualDebugger/VirtualMachineModel.cs
index 8993ed2..ad792c6 100644
--- a/SuperVM.VisualDebugger/VirtualMachineModel.cs
+++ b/SuperVM.VisualDebugger/VirtualMachineModel.cs
@@ -172,7 +172,7 @@ namespace SuperVM.VisualDebugger
 		}
 
 		public StackItem[] Stack => Enumerable
-			.Range(0, (int)this.process.StackPointer)
+			.Range(1, (int)this.process.StackPointer)
 			.Select(i => new StackItem(i, this.process))
 			.ToArray();