Function returning a tuple or None: how to call that function nicely?

Suppose the following:

def MyFunc(a):
  if a < 0:
    return None
  return (a+1, a+2, a+3)

v1, v2, v3 = MyFunc()
# Bad ofcourse, if the result was None

What is the best way to define a function that returns a tuple and yet can be nicely called. Currently, I could do this:

r = MyFunc()
if r:
  v1, v2, v3 = r
else:
  # bad!!
  pass

What I don't like about this is that I have to use a single variable and then unpack it.

Another solution is I could have the function return a tuple full of Nones so that the caller can nicely unpack....

Anyone can suggest a better design?