diff --git a/Lib/test/test_weakref.py b/Lib/test/test_weakref.py
index 9fa0bbd..1fac08d 100644
--- a/Lib/test/test_weakref.py
+++ b/Lib/test/test_weakref.py
@@ -8,6 +8,7 @@
 import copy
 import threading
 import time
+import random
 
 from test import support
 from test.support import script_helper
@@ -1688,6 +1689,87 @@
                 self.assertEqual(len(d), 1)
                 o = None  # lose ref
 
+    def check_threaded_weak_dict_copy(self, type_, deepcopy):
+        # `type_` should be either WeakKeyDictionary or WeakValueDictionary.
+        # `deepcopy` should be either True or False.
+        exc = []
+
+        class DummyKey:
+            def __init__(self, ctr):
+                self.ctr = ctr
+
+        class DummyValue:
+            def __init__(self, ctr):
+                self.ctr = ctr
+
+        def dict_copy(d, exc):
+            try:
+                if deepcopy is True:
+                    _ = copy.deepcopy(d)
+                else:
+                    _ = d.copy()
+            except Exception as ex:
+                exc.append(ex)
+
+        def pop_and_collect(lst):
+            gc_ctr = 0
+            while lst:
+                i = random.randint(0, len(lst) - 1)
+                gc_ctr += 1
+                lst.pop(i)
+                if gc_ctr % 10000 == 0:
+                    gc.collect()  # just in case
+
+        self.assertIn(type_, (weakref.WeakKeyDictionary, weakref.WeakValueDictionary))
+
+        d = type_()
+        keys = []
+        values = []
+        # Initialize d with many entries
+        for i in range(70000):
+            k, v = DummyKey(i), DummyValue(i)
+            keys.append(k)
+            values.append(v)
+            d[k] = v
+            del k
+            del v
+
+        t_copy = threading.Thread(target=dict_copy, args=(d, exc,))
+        if type_ is weakref.WeakKeyDictionary:
+            t_collect = threading.Thread(target=pop_and_collect, args=(keys,))
+        else:  # weakref.WeakValueDictionary
+            t_collect = threading.Thread(target=pop_and_collect, args=(values,))
+
+        t_copy.start()
+        t_collect.start()
+
+        t_copy.join()
+        t_collect.join()
+
+        # Test exceptions
+        if exc:
+            raise exc[0]
+
+    def test_threaded_weak_key_dict_copy(self):
+        # Issue #35615: Weakref keys or values getting GC'ed during dict
+        # copying should not result in a crash.
+        self.check_threaded_weak_dict_copy(weakref.WeakKeyDictionary, False)
+
+    def test_threaded_weak_key_dict_deepcopy(self):
+        # Issue #35615: Weakref keys or values getting GC'ed during dict
+        # copying should not result in a crash.
+        self.check_threaded_weak_dict_copy(weakref.WeakKeyDictionary, True)
+
+    def test_threaded_weak_value_dict_copy(self):
+        # Issue #35615: Weakref keys or values getting GC'ed during dict
+        # copying should not result in a crash.
+        self.check_threaded_weak_dict_copy(weakref.WeakValueDictionary, False)
+
+    def test_threaded_weak_value_dict_deepcopy(self):
+        # Issue #35615: Weakref keys or values getting GC'ed during dict
+        # copying should not result in a crash.
+        self.check_threaded_weak_dict_copy(weakref.WeakValueDictionary, True)
+
 
 from test import mapping_tests
 
diff --git a/Lib/weakref.py b/Lib/weakref.py
index 99de2ea..753f072 100644
--- a/Lib/weakref.py
+++ b/Lib/weakref.py
@@ -171,10 +171,11 @@
         if self._pending_removals:
             self._commit_removals()
         new = WeakValueDictionary()
-        for key, wr in self.data.items():
-            o = wr()
-            if o is not None:
-                new[key] = o
+        with _IterationGuard(self):
+            for key, wr in self.data.items():
+                o = wr()
+                if o is not None:
+                    new[key] = o
         return new
 
     __copy__ = copy
@@ -184,10 +185,11 @@
         if self._pending_removals:
             self._commit_removals()
         new = self.__class__()
-        for key, wr in self.data.items():
-            o = wr()
-            if o is not None:
-                new[deepcopy(key, memo)] = o
+        with _IterationGuard(self):
+            for key, wr in self.data.items():
+                o = wr()
+                if o is not None:
+                    new[deepcopy(key, memo)] = o
         return new
 
     def get(self, key, default=None):
@@ -408,10 +410,11 @@
 
     def copy(self):
         new = WeakKeyDictionary()
-        for key, value in self.data.items():
-            o = key()
-            if o is not None:
-                new[o] = value
+        with _IterationGuard(self):
+            for key, value in self.data.items():
+                o = key()
+                if o is not None:
+                    new[o] = value
         return new
 
     __copy__ = copy
@@ -419,10 +422,11 @@
     def __deepcopy__(self, memo):
         from copy import deepcopy
         new = self.__class__()
-        for key, value in self.data.items():
-            o = key()
-            if o is not None:
-                new[o] = deepcopy(value, memo)
+        with _IterationGuard(self):
+            for key, value in self.data.items():
+                o = key()
+                if o is not None:
+                    new[o] = deepcopy(value, memo)
         return new
 
     def get(self, key, default=None):
diff --git a/Misc/NEWS.d/next/Library/2018-12-30-20-00-05.bpo-35615.Uz1SVh.rst b/Misc/NEWS.d/next/Library/2018-12-30-20-00-05.bpo-35615.Uz1SVh.rst
new file mode 100644
index 0000000..4aff8f7
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-12-30-20-00-05.bpo-35615.Uz1SVh.rst
@@ -0,0 +1,3 @@
+:mod:`weakref`: Fix a RuntimeError when copying a WeakKeyDictionary or a
+WeakValueDictionary, due to some keys or values disappearing while
+iterating.