Covariant Derivative gives Error, why? (SAGE 7.5.1)

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This simple code gives an Error:

f = function('f')

B=Manifold(2,'B',start_index=1)

polar.<R,Phi> = B.chart(R'R:(0,+oo) Phi:(0,2*pi):\Phi')

G = B.riemannian_metric('G')

G[1,1]=diff(f(R),R)

G[2,2]=f(R)^2

nabla=G.connection()

S=B.tensor_field(1,1)

S[1,1]=R^(0.5)

S[2,2]=R^3

S.display()

nabla(S)

Error: TypeError: unable to convert R to an integer

This Error doesn't occur either if I avoid the square root in S[1,1]=R^(0.5), for example by writing S[1,1]=R^(0.4) or if I avoid the dependence of the derivative of f in the first entry of the metric, for example by writing G[1,1]=f(R)^2.

This Error also doesn't occur in older Sage versions, for example Sage 7.1

Thanks a lot for help!