diff --git a/WeaselTSF/CandidateList.cpp b/WeaselTSF/CandidateList.cpp
index cdf2c3e62..b72f8866c 100644
--- a/WeaselTSF/CandidateList.cpp
+++ b/WeaselTSF/CandidateList.cpp
@@ -285,6 +285,10 @@ HRESULT CCandidateList::_UpdateUIElement() {
 
 void CCandidateList::StartUI() {
   com_ptr<ITfThreadMgr> pThreadMgr = _tsf->_GetThreadMgr();
+  if (!pThreadMgr) {
+    return;
+  }
+
   com_ptr<ITfUIElementMgr> pUIElementMgr;
   auto hr = pThreadMgr->QueryInterface(&pUIElementMgr);
   if (FAILED(hr))