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

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

Rick Venable
computational chemist