Server log error (Minecraft)

java.lang.NullPointerException
        at ek.a(SourceFile:231)
        at fh.a(SourceFile:50)
        at ek.a(SourceFile:222)
        at ck.h(SourceFile:182)
        at ck.d(SourceFile:16)
        at cm.run(SourceFile:115)
2013-05-04 00:00:00 [INFO] <username> lost connection: disconnect.genericReason
2013-05-04 00:00:00 [INFO] <username> lost connection: disconnect.genericReason
2013-05-04 00:00:00 [SEVERE] Reached end of stream
2013-05-04 00:00:00 [INFO] /192.168.1.100:47481 lost connection
java.lang.IllegalArgumentException: Empty string not allowed
        at cg.<init>(SourceFile:15)
        at bu.a(SourceFile:77)
        at asa.e(SourceFile:186)
        at asa.b(SourceFile:112)
        at alh.a(SourceFile:88)
        at alh.a(SourceFile:76)
        at jb.a(SourceFile:646)
        at jb.a(SourceFile:631)
        at net.minecraft.server.MinecraftServer.a(SourceFile:265)
        at net.minecraft.server.MinecraftServer.r(SourceFile:412)
        at net.minecraft.server.MinecraftServer.run(SourceFile:341)
        at gr.run(SourceFile:584)