Entering minecart causes null pointer

MUST BE ON SERVER FOR THIS TO HAPPEN!!

Client> 2013-04-18 16:49:37 [CLIENT] [INFO] Connecting to xx.xxx.xxx.xx, 25565
Client> java.lang.NullPointerException
Client> at ek.n(SourceFile:174)
Client> at ck.a(SourceFile:237)
Client> at ck.a(SourceFile:225)
Client> at ck.h(SourceFile:175)
Client> at ck.d(SourceFile:16)
Client> at cm.run(SourceFile:115)
Client> java.io.FileNotFoundException: http://assets.minecraft.net/1_6_has_been_released.flag
Client> at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
Client> at kq.a(SourceFile:237)
Client> at bhq.run(SourceFile:114)
Client> at java.lang.Thread.run(Unknown Source)