Problem using StringVar()

Just trying to learn to use Tkinter.

python 2.7.3  OS Ubuntu 12.4

This code
-----------------------
#!/usr/bin/python
import Tkinter
top = Tkinter.Tk()

v = top.StringVar()
Label(master, textvariable=v).pack()

v.set("hello")

top.mainloop()
---------------------------
returns this error message

Traceback (most recent call last):
   File "./hello.py", line 5, in <module>
     v = top.StringVar()
   File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 1767, in __getattr__
     return getattr(self.tk, attr)
AttributeError: StringVar


Please what am I doing wrong?