symbolic vector norm

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I can't get Sage to produce the norm or even just compute w[1]^2 in the code below. I keep getting a "not implemented" error. Have I made a simple mistake here, or is there a way around this? Thanks for the help. I haven't done much with matrix computations in Sage yet and am trying to learn my way around.

var('x,y')
a=matrix([[1,2],[-3,0]])
v=vector([x,y])
w=a*v.column()
w.norm()