diff --git a/Lib/test/test_peepholer.py b/Lib/test/test_peepholer.py
index 531b425..b7d446f 100644
--- a/Lib/test/test_peepholer.py
+++ b/Lib/test/test_peepholer.py
@@ -267,11 +267,23 @@
         asm = disassemble(f)
         self.assertNotIn('BINARY_ADD', asm)
 
+class TestBuglets(unittest.TestCase):
+
+    def test_bug_11510(self):
+        # folded constant set optimization was commingled with the tuple
+        # unpacking optimization which would fail if the set had duplicate
+        # elements so that the set length was unexpected
+        def f():
+            x, y = {1, 1}
+            return x, y
+        with self.assertRaises(ValueError):
+            f()
+
 
 def test_main(verbose=None):
     import sys
     from test import support
-    test_classes = (TestTranforms,)
+    test_classes = (TestTranforms, TestBuglets)
     support.run_unittest(*test_classes)
 
     # verify reference counting
diff --git a/Misc/NEWS b/Misc/NEWS
index 8b72498..42330ff 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,8 @@
 Core and Builtins
 -----------------
 
+- Issue #11510: Fixed optimizer bug which turned "a,b={1,1}" into "a,b=(1,1)".
+
 - Issue #11432: A bug was introduced in subprocess.Popen on posix systems with
   3.2.0 where the stdout or stderr file descriptor being the same as the stdin
   file descriptor would raise an exception. webbrowser.open would fail. fixed.
diff --git a/Python/peephole.c b/Python/peephole.c
index f972e16..6985043 100644
--- a/Python/peephole.c
+++ b/Python/peephole.c
@@ -475,7 +475,8 @@
                 }
                 if (codestr[i+3] != UNPACK_SEQUENCE  ||
                     !ISBASICBLOCK(blocks,i,6) ||
-                    j != GETARG(codestr, i+3))
+                    j != GETARG(codestr, i+3) ||
+                    opcode == BUILD_SET)
                     continue;
                 if (j == 1) {
                     memset(codestr+i, NOP, 6);