diff --git a/Lib/test/test_string.py b/Lib/test/test_string.py
--- a/Lib/test/test_string.py
+++ b/Lib/test/test_string.py
@@ -111,6 +111,12 @@
 
     def test_formatter(self):
         fmt = string.Formatter()
+
+        self.assertEqual(fmt.format(""), "")
+        self.assertEqual(type(fmt.format("")), str)
+        self.assertEqual(fmt.format(u""), u"")
+        self.assertEqual(type(fmt.format(u"")), unicode)  # Issue #15951
+
         self.assertEqual(fmt.format("foo"), "foo")
 
         self.assertEqual(fmt.format("foo{0}", "bar"), "foobar")