There is a parameter error of sorts, in that you've only read the protein topology and parameter files. From the comments in the file toppar_all36_prot_na_combined.str:
!Parent files that have to be read prior to streaming this file
!top_all36_prot.rtf
!par_all36_prot.prm
!top_all36_na.rtf
!par_all36_na.prm
Also, creating your own custom topology and parameter files by combining existing distributed files is not recommended for the general user. It is best to read them from a standard location, and not change those files in any way
