Index: lldb/trunk/lit/tools/lldb-mi/exec/exec-step.test
===================================================================
--- lldb/trunk/lit/tools/lldb-mi/exec/exec-step.test
+++ lldb/trunk/lit/tools/lldb-mi/exec/exec-step.test
@@ -0,0 +1,30 @@
+# XFAIL: windows
+# -> llvm.org/pr24452
+#
+# RUN: %cc -o %t %p/inputs/main.c -g
+# RUN: %lldbmi %t < %s | FileCheck %s
+
+# Test lldb-mi -exec-step command.
+
+# Check that we have a valid target created via '%lldbmi %t'.
+# CHECK: ^done
+
+-break-insert main
+# CHECK: ^done,bkpt={number="1"
+
+-exec-run
+# CHECK: ^running
+# CHECK: *stopped,reason="breakpoint-hit"
+
+-exec-step --thread 0
+# Check that exec-step can process the case of invalid thread ID.
+# CHECK: ^error,msg="Command 'exec-step'. Thread ID invalid"
+
+-exec-step --thread 1
+# CHECK: ^running
+# CHECK: *stopped,reason="end-stepping-range"
+
+-exec-step
+# Check that exec-step can step-in in a selected thread.
+# CHECK: ^running
+# CHECK: *stopped,reason="end-stepping-range"
Index: lldb/trunk/packages/Python/lldbsuite/test/tools/lldb-mi/control/TestMiExec.py
===================================================================
--- lldb/trunk/packages/Python/lldbsuite/test/tools/lldb-mi/control/TestMiExec.py
+++ lldb/trunk/packages/Python/lldbsuite/test/tools/lldb-mi/control/TestMiExec.py
@@ -330,9 +330,9 @@
 
         # Test that an invalid --thread is handled
         self.runCmd("-exec-step --thread 0")
-        self.expect("\^error,message=\"error: Thread index 0 is out of range")
+        self.expect("\^error,msg=\"Command 'exec-step'. Thread ID invalid")
         self.runCmd("-exec-step --thread 10")
-        self.expect("\^error,message=\"error: Thread index 10 is out of range")
+        self.expect("\^error,msg=\"Command 'exec-step'. Thread ID invalid")
 
         # Test that an invalid --frame is handled
         # FIXME: no error is returned
Index: lldb/trunk/tools/lldb-mi/MICmdCmdExec.h
===================================================================
--- lldb/trunk/tools/lldb-mi/MICmdCmdExec.h
+++ lldb/trunk/tools/lldb-mi/MICmdCmdExec.h
@@ -152,7 +152,6 @@
 
   // Attributes:
 private:
-  lldb::SBCommandReturnObject m_lldbResult;
   const CMIUtilString m_constStrArgNumber; // Not specified in MI spec but
                                            // Eclipse gives this option
 };
Index: lldb/trunk/tools/lldb-mi/MICmdCmdExec.cpp
===================================================================
--- lldb/trunk/tools/lldb-mi/MICmdCmdExec.cpp
+++ lldb/trunk/tools/lldb-mi/MICmdCmdExec.cpp
@@ -487,14 +487,26 @@
 
   CMICmnLLDBDebugSessionInfo &rSessionInfo(
       CMICmnLLDBDebugSessionInfo::Instance());
-  lldb::SBDebugger &rDebugger = rSessionInfo.GetDebugger();
-  CMIUtilString strCmd("thread step-in");
-  if (nThreadId != UINT64_MAX)
-    strCmd += CMIUtilString::Format(" %llu", nThreadId);
-  rDebugger.GetCommandInterpreter().HandleCommand(strCmd.c_str(), m_lldbResult,
-                                                  false);
 
-  return MIstatus::success;
+  lldb::SBError error;
+  if (nThreadId != UINT64_MAX) {
+    lldb::SBThread sbThread =
+        rSessionInfo.GetProcess().GetThreadByIndexID(nThreadId);
+    if (!sbThread.IsValid()) {
+      SetError(CMIUtilString::Format(MIRSRC(IDS_CMD_ERR_THREAD_INVALID),
+                                     m_cmdData.strMiCmd.c_str(),
+                                     m_constStrArgThread.c_str()));
+      return MIstatus::failure;
+    }
+    sbThread.StepInto(nullptr, LLDB_INVALID_LINE_NUMBER, error);
+  } else rSessionInfo.GetProcess().GetSelectedThread().StepInto(
+             nullptr, LLDB_INVALID_LINE_NUMBER, error);
+
+  if (error.Success())
+    return MIstatus::success;
+
+  SetError(error.GetCString());
+  return MIstatus::failure;
 }
 
 //++
@@ -509,21 +521,8 @@
 // Throws:  None.
 //--
 bool CMICmdCmdExecStep::Acknowledge() {
-  if (m_lldbResult.GetErrorSize() > 0) {
-    const char *pLldbErr = m_lldbResult.GetError();
-    MIunused(pLldbErr);
-    const CMICmnMIValueConst miValueConst(m_lldbResult.GetError());
-    const CMICmnMIValueResult miValueResult("message", miValueConst);
-    const CMICmnMIResultRecord miRecordResult(
-        m_cmdData.strMiCmdToken, CMICmnMIResultRecord::eResultClass_Error,
-        miValueResult);
-    m_miResultRecord = miRecordResult;
-  } else {
-    const CMICmnMIResultRecord miRecordResult(
-        m_cmdData.strMiCmdToken, CMICmnMIResultRecord::eResultClass_Running);
-    m_miResultRecord = miRecordResult;
-  }
-
+  m_miResultRecord = CMICmnMIResultRecord(
+      m_cmdData.strMiCmdToken, CMICmnMIResultRecord::eResultClass_Running);
   return MIstatus::success;
 }