diff --git a/agent/src/share/classes/sun/jvm/hotspot/debugger/linux/x86/LinuxX86CFrame.java b/agent/src/share/classes/sun/jvm/hotspot/debugger/linux/x86/LinuxX86CFrame.java
index 53a2e26a84b05dc48763b62d000f201185a052be..95fa38d72a4ac995de013d828e62e9448270556a 100644
--- a/agent/src/share/classes/sun/jvm/hotspot/debugger/linux/x86/LinuxX86CFrame.java
+++ b/agent/src/share/classes/sun/jvm/hotspot/debugger/linux/x86/LinuxX86CFrame.java
@@ -55,7 +55,15 @@ final public class LinuxX86CFrame extends BasicCFrame {
 
    public CFrame sender(ThreadProxy thread) {
       X86ThreadContext context = (X86ThreadContext) thread.getContext();
-      Address esp = context.getRegisterAsAddress(X86ThreadContext.ESP);
+      /*
+       * Native code fills in the stack pointer register value using index
+       * X86ThreadContext.SP.
+       * See file LinuxDebuggerLocal.c macro REG_INDEX(reg).
+       *
+       * Be sure to use SP, or UESP which is aliased to SP in Java code,
+       * for the frame pointer validity check.
+       */
+      Address esp = context.getRegisterAsAddress(X86ThreadContext.SP);
 
       if ( (ebp == null) || ebp.lessThan(esp) ) {
         return null;
diff --git a/agent/src/share/classes/sun/jvm/hotspot/debugger/windows/x86/WindowsX86CFrame.java b/agent/src/share/classes/sun/jvm/hotspot/debugger/windows/x86/WindowsX86CFrame.java
index dbc8b71f35c80ec6da35612167e80f0ef30dc326..1d5e015e8888657ac182dca380840b3e9e3ad155 100644
--- a/agent/src/share/classes/sun/jvm/hotspot/debugger/windows/x86/WindowsX86CFrame.java
+++ b/agent/src/share/classes/sun/jvm/hotspot/debugger/windows/x86/WindowsX86CFrame.java
@@ -46,7 +46,15 @@ public class WindowsX86CFrame extends BasicCFrame {
 
   public CFrame sender(ThreadProxy thread) {
     X86ThreadContext context = (X86ThreadContext) thread.getContext();
-    Address esp = context.getRegisterAsAddress(X86ThreadContext.ESP);
+    /*
+     * Native code fills in the stack pointer register value using index
+     * X86ThreadContext.SP.
+     * See file sawindbg.cpp macro REG_INDEX(x).
+     *
+     * Be sure to use SP, or UESP which is aliased to SP in Java code,
+     * for the frame pointer validity check.
+     */
+    Address esp = context.getRegisterAsAddress(X86ThreadContext.SP);
 
     if ( (ebp == null) || ebp.lessThan(esp) ) {
       return null;