diff --git a/src/api/api_context.cpp b/src/api/api_context.cpp
index aa96998b3..a16341955 100644
--- a/src/api/api_context.cpp
+++ b/src/api/api_context.cpp
@@ -51,11 +51,14 @@ namespace api {
     }
 
     void context::del_object(api::object* o) {
+#ifndef SINGLE_THREAD
         if (m_concurrent_dec_ref) {
             lock_guard lock(m_mux);
             m_objects_to_flush.push_back(o);
         }
-        else {
+        else
+#endif
+        {
             m_free_object_ids.push_back(o->id());
             m_allocated_objects.remove(o->id());
             dealloc(o);
@@ -63,25 +66,26 @@ namespace api {
     }
 
     void context::dec_ref(ast* a) {
+#ifndef SINGLE_THREAD
         if (m_concurrent_dec_ref) {
             lock_guard lock(m_mux);
             m_asts_to_flush.push_back(a);
         }
-        else 
+        else
+#endif
             m().dec_ref(a);
     }
 
     void context::flush_objects() {
+#ifndef SINGLE_THREAD
         if (!m_concurrent_dec_ref)
             return;        
         {
             lock_guard lock(m_mux);
             if (m_asts_to_flush.empty() && m_objects_to_flush.empty())
                 return;
-            m_asts_to_flush2.append(m_asts_to_flush);
-            m_asts_to_flush.reset();
-            m_objects_to_flush2.append(m_objects_to_flush);
-            m_objects_to_flush.reset();
+            m_asts_to_flush2.swap(m_asts_to_flush);
+            m_objects_to_flush2.swap(m_objects_to_flush);
         }
         for (ast* a : m_asts_to_flush2)
             m().dec_ref(a);
@@ -92,7 +96,7 @@ namespace api {
         }
         m_objects_to_flush2.reset();
         m_asts_to_flush2.reset();
-            
+#endif
     }
 
     static void default_error_handler(Z3_context ctx, Z3_error_code c) {
diff --git a/src/api/api_context.h b/src/api/api_context.h
index cf8c6ac87..182958b1e 100644
--- a/src/api/api_context.h
+++ b/src/api/api_context.h
@@ -75,6 +75,9 @@ namespace api {
         struct add_plugins {  add_plugins(ast_manager & m); };
         ast_context_params                m_params;
         bool                       m_user_ref_count; //!< if true, the user is responsible for managing reference counters.
+#ifndef SINGLE_THREAD
+        bool                       m_concurrent_dec_ref = false;
+#endif
         scoped_ptr<ast_manager>    m_manager;
         scoped_ptr<cmd_context>    m_cmd;
         add_plugins                m_plugins;
@@ -91,9 +94,10 @@ namespace api {
         smt_params                 m_fparams;
         // -------------------------------
 
-        bool                       m_concurrent_dec_ref = false;        
+#ifndef SINGLE_THREAD
         ptr_vector<ast>            m_asts_to_flush, m_asts_to_flush2;
         ptr_vector<api::object>    m_objects_to_flush, m_objects_to_flush2;
+#endif
 
         ast_ref_vector             m_ast_trail;        
         ref<api::object>           m_last_obj; //!< reference to the last API object returned by the APIs
@@ -173,7 +177,13 @@ namespace api {
         void set_error_code(Z3_error_code err, std::string &&opt_msg);
         void set_error_handler(Z3_error_handler h) { m_error_handler = h; }
         
-        void enable_concurrent_dec_ref() { m_concurrent_dec_ref = true; }
+        void enable_concurrent_dec_ref() {
+#ifdef SINGLE_THREAD
+            set_error_code(Z3_EXCEPTION, "Can't use concurrent features with a single-thread build");
+#else
+            m_concurrent_dec_ref = true;
+#endif
+        }
         unsigned add_object(api::object* o);
         void del_object(api::object* o);
         void dec_ref(ast* a);
diff --git a/src/api/api_util.h b/src/api/api_util.h
index 474d3410e..80f4f5ec7 100644
--- a/src/api/api_util.h
+++ b/src/api/api_util.h
@@ -19,8 +19,8 @@ Revision History:
 
 #include "util/params.h"
 #include "util/lbool.h"
+#include "util/mutex.h"
 #include "ast/ast.h"
-#include <atomic>
 
 #define Z3_TRY try {
 #define Z3_CATCH_CORE(CODE) } catch (z3_exception & ex) { mk_c(c)->handle_exception(ex); CODE }
@@ -88,7 +88,6 @@ inline lbool    to_lbool(Z3_lbool b) { return static_cast<lbool>(b); }
 struct Z3_params_ref : public api::object {
     params_ref m_params;
     Z3_params_ref(api::context& c): api::object(c) {}
-    ~Z3_params_ref() override {}
 };
 
 inline Z3_params_ref * to_params(Z3_params p) { return reinterpret_cast<Z3_params_ref *>(p); }
@@ -98,7 +97,6 @@ inline params_ref& to_param_ref(Z3_params p) { return p == nullptr ? const_cast<
 struct Z3_param_descrs_ref : public api::object {
     param_descrs m_descrs;
     Z3_param_descrs_ref(api::context& c): api::object(c) {}
-    ~Z3_param_descrs_ref() override {}
 };
 
 inline Z3_param_descrs_ref * to_param_descrs(Z3_param_descrs p) { return reinterpret_cast<Z3_param_descrs_ref *>(p); }