Vector boson decay in FeynRules

Hi!
I was trying to compute width of a vector boson in MG5, but got an error message.
I found out that problem is coming from the FeynRules and, after some experiments with the code, I realized, that the same problem appears when trying to compute decay width even using the Standard Model particles.
The thing is, that if Lagrangian contains contraction of Levi-Civita tensor with 3 vector bosons and 1 derivative, like "Eps[mu, nu, lambda, rho] Z[mu] Wbar[nu] FS[W, lambda, rho]", than I get a result for computation width from this Lagrangian like that:
{Z, SuperDagger[W], W}

(Sqrt[-4
\!\(\*SubsuperscriptBox[\(M\), \(W\), \(2\)]\) MZ^2 + MZ^4] ((-4
\!\(\*SubsuperscriptBox[\(M\), \(W\), \(2\)]\) MZ + MZ^3)^2 +
   Subscript[\[Epsilon], mu$1, PRIVATE`eext[2], Subscript["\[Mu]", 1],
     Subscript["\[Mu]", 3]] Subscript[\[Epsilon], mu$6603,
    PRIVATE`eext[1], PRIVATE`eext[2], PRIVATE`eext[3]]
\!\(\*SubsuperscriptBox[\("p"\), \(1\), \(PRIVATE`eext[1]\)]\)
\!\(\*SubsuperscriptBox[\("p"\), \(1\),
SubscriptBox[\("\[Mu]"\), \(1\)]]\)
\!\(\*SubsuperscriptBox[\("p"\), \(2\), \(mu$1\)]\)
\!\(\*SubsuperscriptBox[\("p"\), \(2\), \(mu$6603\)]\)
\!\(\*SubsuperscriptBox[\("p"\), \(3\), \(PRIVATE`eext[3]\)]\)
\!\(\*SubsuperscriptBox[\("p"\), \(3\),
SubscriptBox[\("\[Mu]"\), \(3\)]]\)))/(12
\!\(\*SubsuperscriptBox[\(M\), \(W\), \(2\)]\) MZ^2 \[Pi] Abs[MZ]^3)

That looks totally not ok, because the answer for the width contains momenta of the particles and Levi-Civita tensors.
I can't find what can "PRIVATE`eext[1]" as index in Mathematica mean.
Sorry, for asking here about FeynRules, but I hope you can help me to find out what is going wrong.
I have the latest version of FeynRules and Wolfram Mathematica 10.3, if it matters for this case.