diff --git a/src/kernel/environment.cpp b/src/kernel/environment.cpp
index ab7ead794..fdf114b31 100644
--- a/src/kernel/environment.cpp
+++ b/src/kernel/environment.cpp
@@ -48,7 +48,7 @@ environment_id::environment_id():m_ptr(new path()), m_depth(0) {}
 environment_id::environment_id(environment_id const & ancestor, bool) {
     if (ancestor.m_depth == std::numeric_limits<unsigned>::max())
         throw exception("maximal depth in is_descendant tree has been reached, use 'forget' method to workaround this limitation");
-    unique_lock<mutex> lock(ancestor.m_ptr->m_mutex);
+    lock_guard<mutex> lock(ancestor.m_ptr->m_mutex);
     if (ancestor.m_ptr->m_next_depth == ancestor.m_depth + 1) {
         m_ptr   = ancestor.m_ptr;
         m_depth = ancestor.m_depth + 1;
diff --git a/src/library/shared_environment.cpp b/src/library/shared_environment.cpp
index b60b76e10..96e90ef47 100644
--- a/src/library/shared_environment.cpp
+++ b/src/library/shared_environment.cpp
@@ -11,27 +11,27 @@ shared_environment::shared_environment() {}
 shared_environment::shared_environment(environment const & env):m_env(env) {}
 
 environment shared_environment::get_environment() const {
-    unique_lock<mutex> l(m_mutex);
+    lock_guard<mutex> l(m_mutex);
     return m_env;
 }
 
 void shared_environment::add(certified_declaration const & d) {
-    unique_lock<mutex> l(m_mutex);
+    lock_guard<mutex> l(m_mutex);
     m_env = m_env.add(d);
 }
 
 void shared_environment::add(declaration const & d) {
-    unique_lock<mutex> l(m_mutex);
+    lock_guard<mutex> l(m_mutex);
     m_env = m_env.add(d);
 }
 
 void shared_environment::replace(certified_declaration const & t) {
-    unique_lock<mutex> l(m_mutex);
+    lock_guard<mutex> l(m_mutex);
     m_env = m_env.replace(t);
 }
 
 void shared_environment::update(std::function<environment(environment const &)> const & f) {
-    unique_lock<mutex> l(m_mutex);
+    lock_guard<mutex> l(m_mutex);
     m_env = f(m_env);
 }
 }