diff --git a/src/util/mpz.h b/src/util/mpz.h
index f55213cc8..dee54b1c8 100644
--- a/src/util/mpz.h
+++ b/src/util/mpz.h
@@ -20,9 +20,7 @@ Revision History:
 #define MPZ_H_
 
 #include<string>
-#ifndef SINGLE_THREAD
-#include<mutex>
-#endif
+#include "util/mutex.h"
 #include "util/util.h"
 #include "util/small_object_allocator.h"
 #include "util/trace.h"