Subsequent use of split gives wrong results

Something rather funny is happening when using w.split() more than once on a Function w,
I think it is best illustrated with the script below, which produces:

These numbers should be the same:
||u|| = 2.0
||u1|| = 1.0

If the "tmp = ..." line is commented out, I get the expected result:

These numbers should be the same:
||u|| = 2.0
||u1|| = 2.0

Anyone else getting this?

--

from dolfin import *

mesh = UnitSquare(2, 2)
V = VectorFunctionSpace(mesh, "CG", 1)

# Define a function
w1 = Function(V)
w1.vector()[:] = 1.0

# Define a second function
w = Function(V)
w.vector()[:] = 2.0

# Extract first component from w1, not used here -- gives expected
# result if this line is commented out, or if deepcopy=False is used
tmp = w1.split()[0]

# Assign w to w1, expecting w and w1 now to be the same
w1.assign(w)

# Extract subcomponents
u1 = w1.split()[0]
u = w.split()[0]

n_u1 = norm(u1)
n_u = norm(u)

print "These numbers should be the same:"
print "||u|| = ", n_u
print "||u1|| = ", n_u1