diff -urN gnome-boxes-3.20.2-orig/src/async-launcher.vala gnome-boxes-3.20.2/src/async-launcher.vala
--- gnome-boxes-3.20.2-orig/src/async-launcher.vala	2016-04-28 10:36:27.000000000 -0400
+++ gnome-boxes-3.20.2/src/async-launcher.vala	2016-11-14 14:24:41.256426153 -0500
@@ -37,6 +37,7 @@
 
         yield;
 
+        thread.join();
         all_threads.remove (thread);
 
         if (e != null)