diff --git a/src/Features/CSharp/Portable/ChangeSignature/CSharpChangeSignatureService.cs b/src/Features/CSharp/Portable/ChangeSignature/CSharpChangeSignatureService.cs
index f67a2f7e2de22ea1c738f32a1a73284ef25a04bc..99d4e5e723854aa068cdd3b1efc779b95a100b6c 100644
--- a/src/Features/CSharp/Portable/ChangeSignature/CSharpChangeSignatureService.cs
+++ b/src/Features/CSharp/Portable/ChangeSignature/CSharpChangeSignatureService.cs
@@ -383,7 +383,7 @@ private SyntaxNode GetNodeContainingTargetNode(SyntaxNode matchingNode)
             if (updatedNode.IsKind(SyntaxKind.InvocationExpression))
             {
                 var invocation = (InvocationExpressionSyntax)updatedNode;
-                var semanticModel = document.GetSemanticModelAsync(cancellationToken).WaitAndGetResult(cancellationToken);
+                var semanticModel = document.GetSemanticModelAsync(cancellationToken).WaitAndGetResult_CanCallOnBackground(cancellationToken);
 
                 var symbolInfo = semanticModel.GetSymbolInfo((InvocationExpressionSyntax)originalNode, cancellationToken);
                 var isReducedExtensionMethod = false;
@@ -466,10 +466,13 @@ private SeparatedSyntaxList<ArgumentSyntax> AddNewArgumentsToList(Document docum
             {
                 if (updatedParameters[i] is AddedParameter addedParameter)
                 {
-                    fullList.Add(SyntaxFactory.Argument(
-                        seenNameEquals ? SyntaxFactory.NameColon(addedParameter.Name) : default,
-                        refKindKeyword: default,
-                        expression: SyntaxFactory.ParseExpression(addedParameter.CallsiteValue)));
+                    if (addedParameter.CallsiteValue != null)
+                    {
+                        fullList.Add(SyntaxFactory.Argument(
+                            seenNameEquals ? SyntaxFactory.NameColon(addedParameter.Name) : default,
+                            refKindKeyword: default,
+                            expression: SyntaxFactory.ParseExpression(addedParameter.CallsiteValue)));
+                    }
                 }
                 else
                 {
diff --git a/src/Features/Core/Portable/ChangeSignature/AbstractChangeSignatureService.cs b/src/Features/Core/Portable/ChangeSignature/AbstractChangeSignatureService.cs
index f96a444943c31c81285985c9ac64a315e28a5742..2bdbabbb726175e97e9277fb112517c4bb0b693c 100644
--- a/src/Features/Core/Portable/ChangeSignature/AbstractChangeSignatureService.cs
+++ b/src/Features/Core/Portable/ChangeSignature/AbstractChangeSignatureService.cs
@@ -61,11 +61,11 @@ public async Task<ImmutableArray<ChangeSignatureCodeAction>> GetChangeSignatureC
 
         internal ChangeSignatureResult ChangeSignature(Document document, int position, Action<string, NotificationSeverity> errorHandler, CancellationToken cancellationToken)
         {
-            var context = GetContextAsync(document, position, restrictToDeclarations: false, cancellationToken: cancellationToken).WaitAndGetResult(cancellationToken);
+            var context = GetContextAsync(document, position, restrictToDeclarations: false, cancellationToken: cancellationToken).WaitAndGetResult_CanCallOnBackground(cancellationToken);
 
             if (context.CanChangeSignature)
             {
-                return ChangeSignatureWithContextAsync(context, cancellationToken).WaitAndGetResult(cancellationToken);
+                return ChangeSignatureWithContextAsync(context, cancellationToken).WaitAndGetResult_CanCallOnBackground(cancellationToken);
             }
             else
             {
@@ -209,7 +209,7 @@ internal ChangeSignatureOptionsResult GetChangeSignatureOptions(ChangeSignatureA
 
             var symbols = FindChangeSignatureReferencesAsync(
                 SymbolAndProjectId.Create(declaredSymbol, context.Project.Id),
-                context.Solution, cancellationToken).WaitAndGetResult(cancellationToken);
+                context.Solution, cancellationToken).WaitAndGetResult_CanCallOnBackground(cancellationToken);
 
             foreach (var symbol in symbols)
             {
diff --git a/src/VisualStudio/Core/Def/Implementation/ChangeSignature/AddParameterDialog.xaml.cs b/src/VisualStudio/Core/Def/Implementation/ChangeSignature/AddParameterDialog.xaml.cs
index d2e3e1d605d4ed93aaead6c97c3681d40c7e2349..de5370c1282bd2b13508f12bad60777fb54cc3d2 100644
--- a/src/VisualStudio/Core/Def/Implementation/ChangeSignature/AddParameterDialog.xaml.cs
+++ b/src/VisualStudio/Core/Def/Implementation/ChangeSignature/AddParameterDialog.xaml.cs
@@ -57,10 +57,11 @@ private void AddParameterDialog_Loaded(object sender, RoutedEventArgs e)
 
         private void OK_Click(object sender, RoutedEventArgs e)
         {
+            ViewModel.TypeName = ((IntellisenseTextBox)TypeContentControl.Content).Text;
+            ViewModel.ParameterName = ((IntellisenseTextBox)NameContentControl.Content).Text;
+
             if (ViewModel.TrySubmit())
             {
-                ViewModel.TypeName = ((IntellisenseTextBox)TypeContentControl.Content).Text;
-                ViewModel.ParameterName = ((IntellisenseTextBox)NameContentControl.Content).Text;
                 DialogResult = true;
             }
         }
diff --git a/src/VisualStudio/Core/Def/Implementation/ChangeSignature/ChangeSignatureDialog.xaml b/src/VisualStudio/Core/Def/Implementation/ChangeSignature/ChangeSignatureDialog.xaml
index 4d2ef31f0891e0496ef685f95b75de53c41b050b..fdc1126635166c507100378eda8720c4c7e130a8 100644
--- a/src/VisualStudio/Core/Def/Implementation/ChangeSignature/ChangeSignatureDialog.xaml
+++ b/src/VisualStudio/Core/Def/Implementation/ChangeSignature/ChangeSignatureDialog.xaml
@@ -277,36 +277,33 @@
                                         Width="16" 
                                         Moniker="{x:Static imagecatalog:KnownMonikers.MoveDown}" 
                                         Grayscale="{Binding IsEnabled, ElementName=DownButton, Converter={StaticResource NegateBooleanConverter}}"/>
-
                 </vs:DialogButton>
                 <vs:DialogButton
-                        Name="RemoveButton"
+                        Name="AddButton"
                         Margin="9 29 0 0"
+                        AutomationProperties.Name="{Binding AddAutomationText}"
+                        Click="Add_Click"
+                        AutomationProperties.AutomationId="AddButton"
+                        Content="{Binding ElementName=dialog, Path=Add}"
+                        Height="Auto" Width="Auto"/>
+                <vs:DialogButton
+                        Name="RemoveButton"
+                        Margin="9 9 0 0"
                         IsEnabled="{Binding CanRemove, Mode=OneWay}" 
                         AutomationProperties.Name="{Binding RemoveAutomationText}"
                         Click="Remove_Click" 
                         AutomationProperties.AutomationId="RemoveButton" 
                         Content="{Binding ElementName=dialog, Path=Remove}" 
-                        Height="Auto" Width="Auto">
-                </vs:DialogButton>
-
+                        Height="Auto" Width="Auto"/>
                 <vs:DialogButton
                         Name="RestoreButton"
-                        Margin="9 9 0 0"
+                        Margin="9 29 0 0"
                         IsEnabled="{Binding CanRestore, Mode=OneWay}" 
                         Click="Restore_Click" 
                         AutomationProperties.AutomationId="RestoreButton" 
                         AutomationProperties.Name="{Binding RestoreAutomationText}"
                         Content="{Binding ElementName=dialog, Path=Restore}" 
                         Height="Auto" Width="Auto"/>
-
-                <vs:DialogButton
-                        Margin="9 29 0 0"
-                        AutomationProperties.Name="{Binding AddAutomationText}"
-                        Click="Add_Click"
-                        AutomationProperties.AutomationId="AddButton"
-                        Content="{Binding ElementName=dialog, Path=Add}"
-                        Height="Auto" Width="Auto"/>
             </StackPanel>
         </Grid>