diff --git a/libguile/threads.c b/libguile/threads.c
index 5d1390941..8b394584e 100644
--- a/libguile/threads.c
+++ b/libguile/threads.c
@@ -300,6 +300,7 @@ guilify_self_1 (SCM_STACKITEM *base)
   pipe (t->sleep_pipe);
   scm_i_pthread_mutex_init (&t->heap_mutex, NULL);
   t->clear_freelists_p = 0;
+  t->gc_running_p = 0;
   t->exited = 0;
 
   t->freelist = SCM_EOL;
diff --git a/libguile/threads.h b/libguile/threads.h
index 3e16fbbcd..89a701075 100644
--- a/libguile/threads.h
+++ b/libguile/threads.h
@@ -69,6 +69,8 @@ typedef struct scm_i_thread {
   */
   SCM freelist, freelist2;
   int clear_freelists_p; /* set if GC was done while thread was asleep */
+  int gc_running_p;      /* non-zero while this thread does GC or a
+			    sweep. */
 
   /* Other thread local things.
    */