diff --git a/frontend/frontend_base.h b/frontend/frontend_base.h
index 381b62c2..5f0e6481 100644
--- a/frontend/frontend_base.h
+++ b/frontend/frontend_base.h
@@ -132,6 +132,9 @@ template <typename FrontendType> struct GenericFrontend
         m.is_toplevel = true;
         m.prefix = "";
         m.path = top;
+
+        std::lock_guard<std::mutex> lock(ctx->mutex);
+
         ctx->top_module = top;
         // Do the actual import, starting from the top level module
         import_module(m, top.str(ctx), top.str(ctx), mod_refs.at(top.str(ctx)));