diff --git a/Signals And Transforms/MainWindow.xaml b/Signals And Transforms/MainWindow.xaml
index d5b8326..9e1a957 100644
--- a/Signals And Transforms/MainWindow.xaml	
+++ b/Signals And Transforms/MainWindow.xaml	
@@ -10,7 +10,7 @@
         xmlns:ribbon="clr-namespace:System.Windows.Controls.Ribbon;assembly=System.Windows.Controls.Ribbon"
         mc:Ignorable="d"
         Title="Signals And Transforms" Height="768" Width="1380">
-    <Grid x:Name="LayoutRoot">
+    <Grid x:Name="LayoutRoot" AllowDrop="True" Drop="LayoutRoot_Drop">
         <Grid.RowDefinitions>
             <RowDefinition Height="Auto"/>
             <RowDefinition Height="Auto"/>
diff --git a/Signals And Transforms/MainWindow.xaml.cs b/Signals And Transforms/MainWindow.xaml.cs
index b41bbcf..99d546b 100644
--- a/Signals And Transforms/MainWindow.xaml.cs	
+++ b/Signals And Transforms/MainWindow.xaml.cs	
@@ -116,10 +116,39 @@ private void MenuItemLoad_Click(object sender, RoutedEventArgs e)
             openFileDialog.Filter = $"{Properties.Resources.DATABASE_FILES} (*{Properties.Resources.WORKBOOK_FILE_EXTENSION})|*{Properties.Resources.WORKBOOK_FILE_EXTENSION}|{Properties.Resources.ALL_FILES} (*.*)|*.*";
             if (openFileDialog.ShowDialog() == true)
             {
-                // Make active
-                WorkBookManager.Manager().Load(openFileDialog.FileName, true);
-                SetActiveWorkbookTitle();
+                SetActiveWorkbook(openFileDialog.FileName);
             }            
         }
+
+        /// <summary>
+        /// Handle the user action of dropping a workbook file onto the app to open. If it 
+        /// isn't a workbook file (ending in the WORKBOOK_FILE_EXTENSION) then it is ignored
+        /// </summary>
+        /// <param name="sender"></param>
+        /// <param name="e"></param>
+        private void LayoutRoot_Drop(object sender, DragEventArgs e)
+        {
+            if (e.Data.GetDataPresent(DataFormats.FileDrop))
+            {
+                // Note that you can have more than one file.
+                string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);
+                string file = files[0];
+                // Make active
+                if (file.EndsWith(Properties.Resources.WORKBOOK_FILE_EXTENSION))
+                {
+                    SetActiveWorkbook(file);
+                }
+            }
+        }
+
+        /// <summary>
+        /// Open the workbook specified in the file pathname variable and set to active workbook
+        /// </summary>
+        /// <param name="file"></param>
+        private void SetActiveWorkbook(string file)
+        {
+            WorkBookManager.Manager().Load(file, true);
+            SetActiveWorkbookTitle();
+        }
     }
 }
diff --git a/Signals And Transforms/Models/WorkBook.cs b/Signals And Transforms/Models/WorkBook.cs
index 8584fe8..5be98d4 100644
--- a/Signals And Transforms/Models/WorkBook.cs	
+++ b/Signals And Transforms/Models/WorkBook.cs	
@@ -56,6 +56,11 @@ public Signal SumOfSources()
         {
             List<Signal> signals = new List<Signal>(Signals.Values);
             Signal baseSignal = signals.Where(sig => sig.Type == SignalType.Source).FirstOrDefault();
+            if (baseSignal == null)
+            {
+                return null;
+            }
+
             Signal workbookSourceSignal = new Signal();
             workbookSourceSignal.Name = "Source";
             workbookSourceSignal.SamplingHZ = baseSignal.SamplingHZ;
diff --git a/Signals And Transforms/View Models/ConvolutionViewModel.cs b/Signals And Transforms/View Models/ConvolutionViewModel.cs
index cea4a15..dc10161 100644
--- a/Signals And Transforms/View Models/ConvolutionViewModel.cs	
+++ b/Signals And Transforms/View Models/ConvolutionViewModel.cs	
@@ -107,6 +107,11 @@ public void PlotData()
         {
             
             Signal workbookSourceSignal = manager.ActiveWorkBook().SumOfSources();
+            if (workbookSourceSignal == null)
+            {
+                return;
+            }
+
             SignalPlotPoints = new List<DataPoint>(workbookSourceSignal.Samples.Count);
 
             for (int idx = 0; idx < workbookSourceSignal.Samples.Count; idx++)
diff --git a/Signals And Transforms/View Models/SignalGeneratorViewModel.cs b/Signals And Transforms/View Models/SignalGeneratorViewModel.cs
index 81ff317..ee31126 100644
--- a/Signals And Transforms/View Models/SignalGeneratorViewModel.cs	
+++ b/Signals And Transforms/View Models/SignalGeneratorViewModel.cs	
@@ -70,7 +70,11 @@ public void PlotSignals()
             FrequencyHistogram = new List<DataPoint>(512);
 
             Signal workbookSourceSignal = workBookManager.ActiveWorkBook().SumOfSources();
-            
+            if (workbookSourceSignal == null)
+            {
+                return;
+            }
+
             // Test data for now
             for (int idx=0; idx<workbookSourceSignal.Samples.Count;idx++)
             {