diff --git a/Lib/weakref.py b/Lib/weakref.py
--- a/Lib/weakref.py
+++ b/Lib/weakref.py
@@ -108,13 +108,13 @@ class WeakValueDictionary(collections.Mu
         def remove(wr, selfref=ref(self)):
             self = selfref()
             if self is not None:
-                if self._iterating:
-                    self._pending_removals.append(wr.key)
-                else:
-                    del self.data[wr.key]
+                # This callback can be called at any point by the GC,
+                # so just queue the key for removal to avoid race conditions
+                # (issue #28427)
+                self._pending_removals.add(wr.key)
         self._remove = remove
-        # A list of keys to be removed
-        self._pending_removals = []
+        # A set of keys to be removed
+        self._pending_removals = set()
         self._iterating = set()
         self.data = d = {}
         self.update(*args, **kw)
@@ -125,9 +125,14 @@ class WeakValueDictionary(collections.Mu
         # We shouldn't encounter any KeyError, because this method should
         # always be called *before* mutating the dict.
         while l:
-            del d[l.pop()]
+            key = l.pop()
+            # An other value with the same key may have been set in the meantime
+            if d[key]() is None:
+                del d[key]
 
     def __getitem__(self, key):
+        if self._pending_removals:
+            self._commit_removals()
         o = self.data[key]()
         if o is None:
             raise KeyError(key)
@@ -140,9 +145,13 @@ class WeakValueDictionary(collections.Mu
         del self.data[key]
 
     def __len__(self):
-        return len(self.data) - len(self._pending_removals)
+        if self._pending_removals:
+            self._commit_removals()
+        return len(self.data)
 
     def __contains__(self, key):
+        if self._pending_removals:
+            self._commit_removals()
         try:
             o = self.data[key]()
         except KeyError:
@@ -158,6 +167,8 @@ class WeakValueDictionary(collections.Mu
         self.data[key] = KeyedRef(value, self._remove, key)
 
     def copy(self):
+        if self._pending_removals:
+            self._commit_removals()
         new = WeakValueDictionary()
         for key, wr in self.data.items():
             o = wr()
@@ -169,6 +180,8 @@ class WeakValueDictionary(collections.Mu
 
     def __deepcopy__(self, memo):
         from copy import deepcopy
+        if self._pending_removals:
+            self._commit_removals()
         new = self.__class__()
         for key, wr in self.data.items():
             o = wr()
@@ -177,6 +190,8 @@ class WeakValueDictionary(collections.Mu
         return new
 
     def get(self, key, default=None):
+        if self._pending_removals:
+            self._commit_removals()
         try:
             wr = self.data[key]
         except KeyError:
@@ -190,6 +205,8 @@ class WeakValueDictionary(collections.Mu
                 return o
 
     def items(self):
+        if self._pending_removals:
+            self._commit_removals()
         with _IterationGuard(self):
             for k, wr in self.data.items():
                 v = wr()
@@ -197,6 +214,8 @@ class WeakValueDictionary(collections.Mu
                     yield k, v
 
     def keys(self):
+        if self._pending_removals:
+            self._commit_removals()
         with _IterationGuard(self):
             for k, wr in self.data.items():
                 if wr() is not None:
@@ -214,10 +233,14 @@ class WeakValueDictionary(collections.Mu
         keep the values around longer than needed.
 
         """
+        if self._pending_removals:
+            self._commit_removals()
         with _IterationGuard(self):
             yield from self.data.values()
 
     def values(self):
+        if self._pending_removals:
+            self._commit_removals()
         with _IterationGuard(self):
             for wr in self.data.values():
                 obj = wr()
@@ -248,11 +271,11 @@ class WeakValueDictionary(collections.Mu
             return o
 
     def setdefault(self, key, default=None):
+        if self._pending_removals:
+            self._commit_removals()
         try:
             wr = self.data[key]
         except KeyError:
-            if self._pending_removals:
-                self._commit_removals()
             self.data[key] = KeyedRef(default, self._remove, key)
             return default
         else:
@@ -287,6 +310,8 @@ class WeakValueDictionary(collections.Mu
         keep the values around longer than needed.
 
         """
+        if self._pending_removals:
+            self._commit_removals()
         return list(self.data.values())