How to obtain eigenvectors in AceGen

I’m trying to obtain eigenvectors of a 3D plastic stretch tensor $C_p$. There is no SMSEigenvectors function, so I tried with Eigenvectors[Cp], but internal error occurs:

Internal error: SMSStrictFunction

Subroutine: SKR Description: {{1.00000,Null^3.0}}

I was guessing it is because $C_p$ is internal variable of the problem and is defined by some other variables, so I also tried with numerical values: Eigenvectors[N[Cp]]. Error is gone, however calculated result is wrong. I took a closer look at it and noticed that N[Cp] does not evaluate $C_P$ correctly, therefore calculated eigenvectors are wrong.

Questions:

1. Why function N[] gives wrong numerical values?
2. Is there another way to calculate eigenvectors?