C-Index for DeepSurv in R

Sorry, but I new in this issue...
I am working with the R library survivalmodels to do the survival analysis with DeepSurv
I am having serious problems calculating the C-Index with Cindex from the library(pec). The C-index value is very low compared to Cox, for example.
Any recommendations to calculate the value of C-Index in R for DeepSurv?
Thank you.