Server error

I am not able to connect to a 1.6 snapshot server.

client side error: "Inernal server error"
server side stacktrace:
2013-04-18 20:45:12 [WARNING] Failed to handle packet for zipron [/192.168.0.1:50944]: u: Sending packet
u: Sending packet
at jj.b(SourceFile:443)
at gw.a(SourceFile:122)
at gw.a(SourceFile:77)
at jh.d(SourceFile:161)
at jh.c(SourceFile:53)
at ih.a(SourceFile:44)
at ig.b(SourceFile:29)
at net.minecraft.server.MinecraftServer.r(SourceFile:488)
at ib.r(SourceFile:258)
at net.minecraft.server.MinecraftServer.q(SourceFile:405)
at net.minecraft.server.MinecraftServer.run(SourceFile:339)
at gr.run(SourceFile:582)
Caused by: java.lang.NullPointerException
at fh.a(SourceFile:66)
at ck.a(SourceFile:159)
at jj.b(SourceFile:426)
... 11 more
2013-04-18 20:45:12 [SEVERE] Reached end of stream