diff --git a/src/solver.cpp b/src/solver.cpp
index 118af4c8d7..298caaf774 100644
--- a/src/solver.cpp
+++ b/src/solver.cpp
@@ -1035,7 +1035,7 @@ void Solver::setMaxTime(double maxtime) {
 void Solver::startTimer() const { simulation_timer_.reset(); }
 
 bool Solver::timeExceeded(int interval) const {
-    static int eval_counter = 0;
+    thread_local static int eval_counter = 0;
 
     // 0 means infinite time
     if (maxtime_.count() == 0)
diff --git a/src/solver_cvodes.cpp b/src/solver_cvodes.cpp
index 0b60304c55..efe50eb8a9 100644
--- a/src/solver_cvodes.cpp
+++ b/src/solver_cvodes.cpp
@@ -1126,7 +1126,8 @@ static int froot(realtype t, N_Vector x, realtype* root, void* user_data) {
     if (model->ne != model->ne_solver) {
         // temporary buffer to store all root function values, not only the ones
         // tracked by the solver
-        static std::vector<realtype> root_buffer(model->ne, 0.0);
+        thread_local static std::vector<realtype> root_buffer(model->ne, 0.0);
+        root_buffer.resize(model->ne);
         model->froot(t, x, root_buffer);
         std::copy_n(root_buffer.begin(), model->ne_solver, root);
     } else {