Unable to replace file in std.cpk (K-On! Houkago Live!!)

[kourm]

I'm trying to replace a file in std.cpk. It extracts fine, but when trying replace, CriPakTools.exe fails with:

$ mono CriPakTools/bin/Debug/CriPakTools.exe ../K-On\!\ Houkago\ Live\!\!/PSP_GAME/USRDIR/std.cpk teatime.bin ../extracted/std.cpk/PSP_GAME/USRDIR/bind/teatime.bin std.cpk
CriPakTools

Based off Falo's code relased on Xentax forums (see readme.txt), modded by Nanashi3 from FuwaNovels.
Insertion code by EsperKnight


Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object
  at CriPakTools.Program.Main (System.String[] args) [0x005d4] in <63025593547a400085707f511166cbdb>:0 
[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object
  at CriPakTools.Program.Main (System.String[] args) [0x005d4] in <63025593547a400085707f511166cbdb>:0 

(The replacement file is simply the original)

A short printf-debugging session led me to believe that entries[i].FileSize in Program.cs:125 isn't properly set a few iterations in (maybe something like null?). But as I have no clue about C# I gave up on fixing it.