bpo-38870: Throw ValueError on invalid yield from usage (GH-17798)

diff --git a/Lib/test/test_unparse.py b/Lib/test/test_unparse.py
index 49767db..e8b0d4b 100644
--- a/Lib/test/test_unparse.py
+++ b/Lib/test/test_unparse.py
@@ -278,6 +278,8 @@
     def test_invalid_set(self):
         self.check_invalid(ast.Set(elts=[]))
 
+    def test_invalid_yield_from(self):
+        self.check_invalid(ast.YieldFrom(value=None))
 
 class DirectoryTestCase(ASTTestCase):
     """Test roundtrip behaviour on all files in Lib and Lib/test."""