diff --git a/Lib/csv.py b/Lib/csv.py
index 98480ba..fc080cc 100644
--- a/Lib/csv.py
+++ b/Lib/csv.py
@@ -140,8 +140,8 @@
         if self.extrasaction == "raise":
             wrong_fields = [k for k in rowdict if k not in self.fieldnames]
             if wrong_fields:
-                raise ValueError("dict contains fields not in fieldnames: " +
-                                 ", ".join(wrong_fields))
+                raise ValueError("dict contains fields not in fieldnames: "
+                                 + ", ".join([repr(x) for x in wrong_fields]))
         return [rowdict.get(key, self.restval) for key in self.fieldnames]
 
     def writerow(self, rowdict):
diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index d3c1dec..5c95fe3 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -630,6 +630,23 @@
         fileobj = StringIO()
         self.assertRaises(TypeError, csv.DictWriter, fileobj)
 
+    def test_write_fields_not_in_fieldnames(self):
+        fd, name = tempfile.mkstemp()
+        fileobj = os.fdopen(fd, "w+b")
+        try:
+            writer = csv.DictWriter(fileobj, fieldnames = ["f1", "f2", "f3"])
+            # Of special note is the non-string key (issue 19449)
+            with self.assertRaises(ValueError) as cx:
+                writer.writerow({"f4": 10, "f2": "spam", 1: "abc"})
+            exception = str(cx.exception)
+            self.assertIn("fieldnames", exception)
+            self.assertIn("'f4'", exception)
+            self.assertNotIn("'f2'", exception)
+            self.assertIn("1", exception)
+        finally:
+            fileobj.close()
+            os.unlink(name)
+
     def test_read_dict_fields(self):
         fd, name = tempfile.mkstemp()
         fileobj = os.fdopen(fd, "w+b")