diff -r 651aa21433ba Lib/test/test_weakref.py
--- a/Lib/test/test_weakref.py	Mon Feb 02 17:47:31 2015 -0500
+++ b/Lib/test/test_weakref.py	Tue Feb 03 00:36:14 2015 +0100
@@ -1594,6 +1594,34 @@
         dict = weakref.WeakKeyDictionary()
         self.assertRegex(repr(dict), '<WeakKeyDictionary at 0x.*>')
 
+    def test_issue19542(self):
+        import _thread, gc, time
+
+        def f():
+            while running:
+                time.sleep(0.01)
+                gc.collect()
+
+        class C: pass
+        class D: pass
+        d = weakref.WeakValueDictionary()
+
+        running = True
+        _thread.start_new_thread(f, ())
+
+        i = 10**5
+        while i > 0:
+            i -= 1
+            x = d.setdefault(10, D())
+            assert x is not None   # we never put None in there!
+            tp = x.__class__
+            del x
+            if tp is D:
+                c = C(); c.cycle = c
+                d[10] = c
+                c = None
+        running = False
+
 from test import mapping_tests
 
 class WeakValueDictionaryTestCase(mapping_tests.BasicTestMappingProtocol):
diff -r 651aa21433ba Lib/weakref.py
--- a/Lib/weakref.py	Mon Feb 02 17:47:31 2015 -0500
+++ b/Lib/weakref.py	Tue Feb 03 00:36:14 2015 +0100
@@ -233,24 +233,26 @@
         try:
             o = self.data.pop(key)()
         except KeyError:
+            o = None
+        if o is None:
             if args:
                 return args[0]
-            raise
-        if o is None:
             raise KeyError(key)
         else:
             return o
 
     def setdefault(self, key, default=None):
         try:
-            wr = self.data[key]
+            o = self.data[key]()
         except KeyError:
+            o = None
+        if o is None:
             if self._pending_removals:
                 self._commit_removals()
             self.data[key] = KeyedRef(default, self._remove, key)
             return default
         else:
-            return wr()
+            return o
 
     def update(self, dict=None, **kwargs):
         if self._pending_removals: