diff --git a/src/perfect/perfect_adaptor.cpp b/src/perfect/perfect_adaptor.cpp
index 4dd605fda..15de55e17 100644
--- a/src/perfect/perfect_adaptor.cpp
+++ b/src/perfect/perfect_adaptor.cpp
@@ -32,7 +32,6 @@
 
 static Move malom_remove_move = MOVE_NONE;
 
-static std::mutex mtx;
 static std::condition_variable cv;
 
 int GetBestMove(int whiteBitboard, int blackBitboard, int whiteStonesToPlace,