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A problem on Groebner Fans with more than 10 variables

sage: X=[var('x%d' % i) for i in range(20)]

Up to 10 variables, everything is alright:

sage: R=PolynomialRing(QQ,"x",10)
sage: R.ideal(x0).groebner_fan()
Groebner fan of the ideal:
Ideal (x0) of Multivariate Polynomial Ring in x0, x1, x2, x3, x4, x5, x6, x7, x8, x9 over Rational Field

But with at least one more variable, there is a problem:

sage: R=PolynomialRing(QQ,"x",11)
sage: R.ideal(x0).groebner_fan()
RuntimeError                              Traceback (most recent call last)
<ipython-input-49-29624ce56396> in <module>()
----> 1 R.ideal(x0).groebner_fan()

/home/sebastien/sage/local/lib/python2.7/site-packages/sage/rings/polynomial/multi_polynomial_ideal.pyc in groebner_fan(self, is_groebner_basis, symmetry, verbose)
   3486         import sage.rings.polynomial.groebner_fan as groebner_fan
   3487         return groebner_fan.GroebnerFan(self, is_groebner_basis=is_groebner_basis,
-> 3488                                         symmetry=symmetry, verbose=verbose)
   3490     @cached_method

/home/sebastien/sage/local/lib/python2.7/site-packages/sage/rings/polynomial/groebner_fan.pyc in __init__(self, I, is_groebner_basis, symmetry, verbose)
    810             raise TypeError, "I must be a multivariate polynomial ideal"
    811         if prefix_check([str(R_gen) for R_gen in I.ring().gens()]) != True:
--> 812             raise RuntimeError, "Ring variables cannot contain each other as prefixes"
    813         S = I.ring()
    814         R = S.base_ring()

RuntimeError: Ring variables cannot contain each other as prefixes

What's the problem ? How have more than 10 variables ?