diff --git a/src/AddIns/Misc/Debugger/Debugger.Core/Project/Src/Debugger/Process-StateControl.cs b/src/AddIns/Misc/Debugger/Debugger.Core/Project/Src/Debugger/Process-StateControl.cs
index b008bb1742..3903c9623b 100644
--- a/src/AddIns/Misc/Debugger/Debugger.Core/Project/Src/Debugger/Process-StateControl.cs
+++ b/src/AddIns/Misc/Debugger/Debugger.Core/Project/Src/Debugger/Process-StateControl.cs
@@ -151,16 +151,19 @@ namespace Debugger
 		
 		public void StepInto()
 		{
+			AssertPaused();
 			SelectedStackFrame.StepInto();
 		}
 		
 		public void StepOver()
 		{
+			AssertPaused();
 			SelectedStackFrame.StepOver();
 		}
 		
 		public void StepOut()
 		{
+			AssertPaused();
 			SelectedStackFrame.StepOut();
 		}
 	}
diff --git a/src/AddIns/Misc/Debugger/Debugger.Core/Project/Src/Threads/Thread.cs b/src/AddIns/Misc/Debugger/Debugger.Core/Project/Src/Threads/Thread.cs
index 8066240eab..38f5f77061 100644
--- a/src/AddIns/Misc/Debugger/Debugger.Core/Project/Src/Threads/Thread.cs
+++ b/src/AddIns/Misc/Debugger/Debugger.Core/Project/Src/Threads/Thread.cs
@@ -33,7 +33,7 @@ namespace Debugger
 		bool hasExpired = false;
 		bool nativeThreadExited = false;
 
-		StackFrame selectedFunction;
+		StackFrame selectedStackFrame;
 		
 		public event EventHandler Expired;
 		public event EventHandler<ThreadEventArgs> NativeThreadExited;
@@ -265,14 +265,15 @@ namespace Debugger
 		[Debugger.Tests.ToStringOnly]
 		public StackFrame SelectedStackFrame {
 			get {
-				return selectedFunction;
+				// Forum-20456: Do not return expired StackFrame
+				if (selectedStackFrame.HasExpired) return null;
+				return selectedStackFrame;
 			}
 			set {
 				if (value != null && !value.HasSymbols) {
 					throw new DebuggerException("SelectedFunction must have symbols");
 				}
-				
-				selectedFunction = value;
+				selectedStackFrame = value;
 			}
 		}