diff --git a/Lib/test/test_set.py b/Lib/test/test_set.py
index 872e9ead98d..bc7465a5ba3 100644
--- a/Lib/test/test_set.py
+++ b/Lib/test/test_set.py
@@ -81,6 +81,10 @@ class TestJointOps(unittest.TestCase):
             self.assertEqual(self.thetype('abcba').union(C('ef')), set('abcef'))
             self.assertEqual(self.thetype('abcba').union(C('ef'), C('fg')), set('abcefg'))
 
+        # Issue #6573
+        x = self.thetype()
+        self.assertEqual(x.union(set([1]), x, set([2])), self.thetype([1, 2]))
+
     def test_or(self):
         i = self.s.union(self.otherword)
         self.assertEqual(self.s | set(self.otherword), i)
diff --git a/Misc/NEWS b/Misc/NEWS
index b6ccc092070..25bbabb72cd 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -14,6 +14,9 @@ Core and Builtins
 
 - Issue #6540: Fixed crash for bytearray.translate() with invalid parameters.
 
+- Issue #6573: set.union() stopped processing inputs if an instance of self
+  occurred in the argument chain.
+
 - Issue #1616979: Added the cp720 (Arabic DOS) encoding.
 
 - Issue #6070: On posix platforms import no longer copies the execute bit
diff --git a/Objects/setobject.c b/Objects/setobject.c
index a55bbb70adb..dd45380a35f 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -1183,7 +1183,7 @@ set_union(PySetObject *so, PyObject *args)
 	for (i=0 ; i<PyTuple_GET_SIZE(args) ; i++) {
 		other = PyTuple_GET_ITEM(args, i);
 		if ((PyObject *)so == other)
-			return (PyObject *)result;
+			continue;
 		if (set_update_internal(result, other) == -1) {
 			Py_DECREF(result);
 			return NULL;