Question: VectorCalculus[TangentPlane]

with(VectorCalculus):

TangentPlane( x^2+y^2, x=a,y=b);

Error, (in VectorCalculus:-TangentPlane) 2 independent variables 
are required

However this is copied from the Help, last example, (2.8). What wrong in this example? Thanks, Sandor
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