diff --git a/Lib/threading.py b/Lib/threading.py
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -1066,7 +1066,8 @@
                 thread._ident = ident
                 # Any condition variables hanging off of the active thread may
                 # be in an invalid state, so we reinitialize them.
-                thread._block.__init__()
+                if hasattr(thread, "_block"):  # Check for _DummyThread
+                    thread._block.__init__()
                 thread._started._cond.__init__()
                 new_active[ident] = thread
             else: