diff --git a/NineChess/src/aithread.cpp b/NineChess/src/aithread.cpp
index 78de87bf..a0c749de 100644
--- a/NineChess/src/aithread.cpp
+++ b/NineChess/src/aithread.cpp
@@ -132,9 +132,10 @@ void AiThread::run()
             QTimer::singleShot(EMIT_COMMAND_DELAY, this, &AiThread::emitCommand);
         } else {
             strCommand = ai_ab.bestMove();
-            qDebug() << "Computer:" << strCommand << "\n";
-            if (strcmp(strCommand, "error!") != 0)
+            if (strCommand && strcmp(strCommand, "error!") != 0) {
+                qDebug() << "Computer:" << strCommand << "\n";
                 QTimer::singleShot(EMIT_COMMAND_DELAY, this, &AiThread::emitCommand);
+            }
         }
 
 #ifdef DEBUG_MODE
diff --git a/NineChess/src/ninechessai_ab.cpp b/NineChess/src/ninechessai_ab.cpp
index fcc38464..23710752 100644
--- a/NineChess/src/ninechessai_ab.cpp
+++ b/NineChess/src/ninechessai_ab.cpp
@@ -1113,6 +1113,10 @@ const char* NineChessAi_ab::bestMove()
 #endif
     //qDebug() << "sizeof(Node) = " << sizeof(Node);
 
+    if (bestMoves.empty()) {
+        return nullptr;
+    }
+
     return move2string(bestMoves[0]->move);
 }