rnfinit error

[Mak Trifkovic <mak@math.mcgill.ca>]

I do the following:

K=bnfinit(a^4 - 3*a^2 + 71);

L=rnfinit(K,x^5 + Mod(a - 1, a^4 - 3*a^2 + 71)*x^4 + Mod(2/5*a^2 + 2/5, 
a^4 - 3*a^2 + 71)*x^3 + Mod(2/5*a^2 - 3/5, a^4 - 3*a^2 + 71)*x^2 + 
Mod(1/5*a^3 - 1/5*a^2 - 4/5*a + 19/5, a^4 - 3*a^2 + 71)*x + Mod(-1/5*a^2 + 
a - 11/5, a^4 - 3*a^2 + 71))

and get the error message

  *** rnfinit: reducible modulus in factornf.

What's wrong?  (both polynomials are irreducible)

Mak