Fix #815

flask/app.py

@@ -949,6 +949,9 @@ class Flask(_PackageBoundObject):
         # a tuple of only `GET` as default.
         if methods is None:
             methods = getattr(view_func, 'methods', None) or ('GET',)
+        if isinstance(methods, string_types):
+            raise TypeError('Allowed methods have to be iterables of strings, '
+                            'for example: @app.route(..., methods=["POST"])')
         methods = set(methods)
 
         # Methods that should always be added