Index: lit/tools/lldb-mi/exec/exec-step.test
===================================================================
--- /dev/null
+++ lit/tools/lldb-mi/exec/exec-step.test
@@ -0,0 +1,26 @@
+# 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
+# Check that exec-step can step-in in a selected thread.
+# CHECK: ^running
+# CHECK: *stopped,reason="end-stepping-range"
Index: tools/lldb-mi/MICmdCmdExec.cpp
===================================================================
--- tools/lldb-mi/MICmdCmdExec.cpp
+++ tools/lldb-mi/MICmdCmdExec.cpp
@@ -510,14 +510,28 @@
 
   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().GetThreadByID(nThreadId);
+    if (sbThread.IsValid())
+      sbThread.StepInto(nullptr, LLDB_INVALID_LINE_NUMBER, error);
+    else {
+      SetError(CMIUtilString::Format(MIRSRC(IDS_CMD_ERR_THREAD_INVALID),
+                                     m_cmdData.strMiCmd.c_str(),
+                                     m_constStrArgThread.c_str()));
+      return MIstatus::failure;
+    }
+  } else rSessionInfo.GetProcess().GetSelectedThread().StepInto(
+             nullptr, LLDB_INVALID_LINE_NUMBER, error);
+
+  if (error.Success())
+    return MIstatus::success;
+
+  SetError(CMIUtilString::Format(MIRSRC(IDS_CMD_ERR_FNFAILED),
+                                 m_cmdData.strMiCmd.c_str(),
+                                 error.GetCString()));
+  return MIstatus::failure;
 }
 
 //++