diff --git a/Lib/test/test_warnings.py b/Lib/test/test_warnings.py
index ca27519..d093994 100644
--- a/Lib/test/test_warnings.py
+++ b/Lib/test/test_warnings.py
@@ -327,6 +327,19 @@
                             self.module.warn_explicit,
                             None, Warning, None, 1, registry=42)
 
+    def test_bad_str(self):
+        # issue 6415
+        # Warnings instance with a bad format string for __str__ should not
+        # trigger a bus error.
+        class BadStrWarning(Warning):
+            """Warning with a bad format string for __str__."""
+            def __str__(self):
+                return ("A bad formatted string %(err)" %
+                        {"err" : "there is no %(err)s"})
+
+        with self.assertRaises(ValueError):
+            self.module.warn(BadStrWarning())
+
 
 class CWarnTests(BaseTest, WarnTests):
     module = c_warnings
diff --git a/Python/_warnings.c b/Python/_warnings.c
index 219cfc6..18fcdac 100644
--- a/Python/_warnings.c
+++ b/Python/_warnings.c
@@ -317,6 +317,8 @@
     }
     if (rc == 1) {
         text = PyObject_Str(message);
+        if (text == NULL)
+            goto cleanup;
         category = (PyObject*)message->ob_type;
     }
     else {