diff --git a/libguile/threads.c b/libguile/threads.c
index dba5d16f1..c07c85342 100644
--- a/libguile/threads.c
+++ b/libguile/threads.c
@@ -1371,6 +1371,8 @@ fat_mutex_lock (SCM mutex, scm_t_timespec *timeout, SCM owner, int *ret)
 	  if (SCM_I_IS_THREAD (new_owner))
 	    {
 	      scm_i_thread *t = SCM_I_THREAD_DATA (new_owner);
+
+	      scm_i_pthread_mutex_unlock (&m->lock);
 	      scm_i_pthread_mutex_lock (&t->admin_mutex);
 
 	      /* Only keep a weak reference to MUTEX so that it's not
@@ -1381,6 +1383,7 @@ fat_mutex_lock (SCM mutex, scm_t_timespec *timeout, SCM owner, int *ret)
 	      t->mutexes = scm_weak_car_pair (mutex, t->mutexes);
 
 	      scm_i_pthread_mutex_unlock (&t->admin_mutex);
+	      scm_i_pthread_mutex_lock (&m->lock);
 	    }
 	  *ret = 1;
 	  break;