diff --git a/sortedcontainers/sortedlist.py b/sortedcontainers/sortedlist.py
index 3b3cd07b..e3b58eb9 100644
--- a/sortedcontainers/sortedlist.py
+++ b/sortedcontainers/sortedlist.py
@@ -339,7 +339,8 @@ def update(self, iterable):
 
         if _maxes:
             if len(values) * 4 >= self._len:
-                values.extend(chain.from_iterable(_lists))
+                _lists.append(values)
+                values = reduce(iadd, _lists, [])
                 values.sort()
                 self._clear()
             else:
@@ -1878,7 +1879,8 @@ def update(self, iterable):
 
         if _maxes:
             if len(values) * 4 >= self._len:
-                values.extend(chain.from_iterable(_lists))
+                _lists.append(values)
+                values = reduce(iadd, _lists, [])
                 values.sort(key=self._key)
                 self._clear()
             else:
diff --git a/tests/test_coverage_sortedkeylist_modulo.py b/tests/test_coverage_sortedkeylist_modulo.py
index 1916054f..5d2cf740 100644
--- a/tests/test_coverage_sortedkeylist_modulo.py
+++ b/tests/test_coverage_sortedkeylist_modulo.py
@@ -5,6 +5,7 @@
 import random
 from .context import sortedcontainers
 from sortedcontainers import SortedList, SortedKeyList
+from itertools import chain, repeat
 import pytest
 
 if hexversion < 0x03000000:
@@ -107,6 +108,45 @@ def test_update():
     assert len(slt) == 11000
     slt._check()
 
+def test_update_order_consistency():
+    def modulo_el0(tup):
+        return tup[0] % 10
+
+    slt1 = SortedKeyList(key=modulo_el0)
+    slt2 = SortedKeyList(key=modulo_el0)
+
+    def add_from_iterable(slt, it):
+        for item in it:
+            slt.add(item)
+
+    def add_from_all_iterables(slt, its):
+        for it in its:
+            add_from_iterable(slt, it)
+
+    def update_from_all_iterables(slt, its):
+        for it in its:
+            slt.update(it)
+
+    # the following iterators are set up (from large to small) such that they
+    # attempt to force the two kinds of internal update logic (extending upon
+    # the incoming iterable or appending to the existing elements by use of
+    # `add()`)
+    it1 = list(zip(repeat(0), range(5)))
+    it2 = list(zip(repeat(0), range(4)))
+    it3 = list(zip(repeat(0), range(3)))
+    it4 = list(zip(repeat(0), range(2)))
+    it5 = list(zip(repeat(0), range(1)))
+
+    it12345 = [it1, it2, it3, it4, it5]
+
+    add_from_all_iterables(slt1, it12345)
+    update_from_all_iterables(slt2, it12345)
+
+    slt1._check()
+    slt2._check()
+
+    assert all(tup[0] == tup[1] for tup in zip(slt1, slt2))
+
 def test_contains():
     slt = SortedKeyList(key=modulo)
     slt._reset(7)
diff --git a/tests/test_coverage_sortedkeylist_negate.py b/tests/test_coverage_sortedkeylist_negate.py
index 6c0287f8..dea380db 100644
--- a/tests/test_coverage_sortedkeylist_negate.py
+++ b/tests/test_coverage_sortedkeylist_negate.py
@@ -5,7 +5,7 @@
 import random
 from .context import sortedcontainers
 from sortedcontainers import SortedKeyList, SortedListWithKey
-from itertools import chain
+from itertools import chain, repeat
 import pytest
 
 if hexversion < 0x03000000:
@@ -84,6 +84,45 @@ def test_update():
     values = sorted((val for val in chain(range(100), range(1000), range(10000))), key=negate)
     assert all(tup[0] == tup[1] for tup in zip(slt, values))
 
+def test_update_order_consistency():
+    def negate_el0(tup):
+        return -tup[0]
+
+    slt1 = SortedKeyList(key=negate_el0)
+    slt2 = SortedKeyList(key=negate_el0)
+
+    def add_from_iterable(slt, it):
+        for item in it:
+            slt.add(item)
+
+    def add_from_all_iterables(slt, its):
+        for it in its:
+            add_from_iterable(slt, it)
+
+    def update_from_all_iterables(slt, its):
+        for it in its:
+            slt.update(it)
+
+    # the following iterators are set up (from large to small) such that they
+    # attempt to force the two kinds of internal update logic (extending upon
+    # the incoming iterable or appending to the existing elements by use of
+    # `add()`)
+    it1 = list(zip(repeat(0), range(5)))
+    it2 = list(zip(repeat(0), range(4)))
+    it3 = list(zip(repeat(0), range(3)))
+    it4 = list(zip(repeat(0), range(2)))
+    it5 = list(zip(repeat(0), range(1)))
+
+    it12345 = [it1, it2, it3, it4, it5]
+
+    add_from_all_iterables(slt1, it12345)
+    update_from_all_iterables(slt2, it12345)
+
+    slt1._check()
+    slt2._check()
+
+    assert all(tup[0] == tup[1] for tup in zip(slt1, slt2))
+
 def test_contains():
     slt = SortedKeyList(key=negate)
     assert 0 not in slt