diff --git a/src/api/api_solver.cpp b/src/api/api_solver.cpp
index 373d49d19..de4d3732f 100644
--- a/src/api/api_solver.cpp
+++ b/src/api/api_solver.cpp
@@ -113,12 +113,12 @@ extern "C" {
     }
 
     void Z3_solver_ref::set_eh(event_handler* eh) {
-        std::lock_guard<std::mutex> lock(m_mux);
+        lock_guard lock(m_mux);
         m_eh = eh;
     }
 
     void Z3_solver_ref::set_cancel() {
-        std::lock_guard<std::mutex> lock(m_mux);
+        lock_guard lock(m_mux);
         if (m_eh) (*m_eh)(API_INTERRUPT_EH_CALLER);
     }
 
diff --git a/src/api/api_solver.h b/src/api/api_solver.h
index 519859cca..c22189857 100644
--- a/src/api/api_solver.h
+++ b/src/api/api_solver.h
@@ -18,6 +18,7 @@ Revision History:
 #ifndef API_SOLVER_H_
 #define API_SOLVER_H_
 
+#include "util/mutex.h"
 #include "api/api_util.h"
 #include "solver/solver.h"
 
@@ -44,7 +45,7 @@ struct Z3_solver_ref : public api::object {
     params_ref                 m_params;
     symbol                     m_logic;
     scoped_ptr<solver2smt2_pp> m_pp;
-    std::mutex                 m_mux;
+    mutex                      m_mux;
     event_handler*             m_eh;
 
     Z3_solver_ref(api::context& c, solver_factory * f):