diff --git a/scp.c b/scp.c
index 094a544d..78adc58a 100644
--- a/scp.c
+++ b/scp.c
@@ -326,6 +326,7 @@
 #if defined (SIM_ASYNCH_IO)
 pthread_mutex_t sim_asynch_lock = PTHREAD_MUTEX_INITIALIZER;
 pthread_cond_t sim_asynch_wake = PTHREAD_COND_INITIALIZER;
+pthread_mutex_t sim_idle_lock = PTHREAD_MUTEX_INITIALIZER;
 
 pthread_mutex_t sim_timer_lock     = PTHREAD_MUTEX_INITIALIZER;
 pthread_cond_t sim_timer_wake      = PTHREAD_COND_INITIALIZER;
diff --git a/sim_defs.h b/sim_defs.h
index aeab12fb..af1f09ed 100644
--- a/sim_defs.h
+++ b/sim_defs.h
@@ -1062,6 +1062,7 @@ struct MEMFILE {
 
 extern pthread_mutex_t sim_asynch_lock;
 extern pthread_cond_t sim_asynch_wake;
+extern pthread_mutex_t sim_idle_lock;
 extern pthread_mutex_t sim_timer_lock;
 extern pthread_cond_t sim_timer_wake;
 extern t_bool sim_timer_event_canceled;
@@ -1174,6 +1175,7 @@ extern int32 sim_asynch_inst_latency;
     do {                                                          \
       pthread_mutex_destroy(&sim_asynch_lock);                    \
       pthread_cond_destroy(&sim_asynch_wake);                     \
+      pthread_mutex_destroy(&sim_idle_lock);                      \
       pthread_mutex_destroy(&sim_timer_lock);                     \
       pthread_cond_destroy(&sim_timer_wake);                      \
       pthread_mutex_destroy(&sim_tmxr_poll_lock);                 \
@@ -1223,6 +1225,7 @@ extern int32 sim_asynch_inst_latency;
     do {                                                          \
       pthread_mutex_destroy(&sim_asynch_lock);                    \
       pthread_cond_destroy(&sim_asynch_wake);                     \
+      pthread_mutex_destroy(&sim_idle_lock);                      \
       pthread_mutex_destroy(&sim_timer_lock);                     \
       pthread_cond_destroy(&sim_timer_wake);                      \
       pthread_mutex_destroy(&sim_tmxr_poll_lock);                 \
diff --git a/sim_timer.c b/sim_timer.c
index 5209338b..2094499d 100644
--- a/sim_timer.c
+++ b/sim_timer.c
@@ -248,9 +248,9 @@ if (done_time.tv_nsec > 1000000000) {
   done_time.tv_sec += done_time.tv_nsec/1000000000;
   done_time.tv_nsec = done_time.tv_nsec%1000000000;
   }
-pthread_mutex_lock (&sim_asynch_lock);
+pthread_mutex_lock (&sim_idle_lock);
 sim_idle_wait = TRUE;
-stat = pthread_cond_timedwait (&sim_asynch_wake, &sim_asynch_lock, &done_time);
+stat = pthread_cond_timedwait (&sim_asynch_wake, &sim_idle_lock, &done_time);
 if (stat == 0)
   sim_asynch_check = 0;                 /* force check of asynch queue now */
 else
@@ -264,7 +264,7 @@ else
     abort ();
     }
 sim_idle_wait = FALSE;
-pthread_mutex_unlock (&sim_asynch_lock);
+pthread_mutex_unlock (&sim_idle_lock);
 if (!timedout) {
     AIO_UPDATE_QUEUE;
     }