diff --git a/src/sat/smt/sls_solver.h b/src/sat/smt/sls_solver.h
index 55e98fac7..5a6c9950b 100644
--- a/src/sat/smt/sls_solver.h
+++ b/src/sat/smt/sls_solver.h
@@ -17,6 +17,7 @@ Author:
 #pragma once
 
 #include <thread>
+#include <mutex>
 #include "util/rlimit.h"
 #include "ast/sls/bv_sls.h"
 #include "sat/smt/sat_th.h"