diff --git a/channels/chan_dahdi.c b/channels/chan_dahdi.c
index 1dc6d06e81..d1fe75d88f 100644
--- a/channels/chan_dahdi.c
+++ b/channels/chan_dahdi.c
@@ -5546,7 +5546,9 @@ static void *ss_thread(void *data)
 	int res;
 	int index;
 
+	ast_mutex_lock(&ss_thread_lock);
 	ss_thread_count++;
+	ast_mutex_unlock(&ss_thread_lock);
 	/* in the bizarre case where the channel has become a zombie before we
 	   even get started here, abort safely
 	*/