diff --git a/src/smt/smt_parallel.cpp b/src/smt/smt_parallel.cpp
index 4d9f84c02..66e4ee28d 100644
--- a/src/smt/smt_parallel.cpp
+++ b/src/smt/smt_parallel.cpp
@@ -341,6 +341,7 @@ namespace smt {
 
     // currenly, the code just implements the greedy strategy
     void parallel::batch_manager::return_cubes(ast_translation& l2g, vector<expr_ref_vector>const& C_worker, expr_ref_vector const& A_worker) {
+        std::scoped_lock lock(mux);
         auto atom_in_cube = [&](expr_ref_vector const& cube, expr* atom) {
             return any_of(cube, [&](expr* e) { return e == atom || (m.is_not(e, e) && e == atom); });
         };