Crash when i try to play multiplayer

---- Minecraft Crash Report ----
// Don't do that.

Time: 24/11/13 15:03
Description: Ticking screen

java.lang.NullPointerException
	at java.net.URLEncoder.encode(Unknown Source)
	at bcw.a(SourceFile:136)
	at bcw.a(SourceFile:123)
	at bcw.a(SourceFile:110)
	at fj.a(SourceFile:41)
	at co.b(SourceFile:354)
	at bcw.e(SourceFile:98)
	at bcy.c(SourceFile:71)
	at atv.k(SourceFile:1173)
	at atv.S(SourceFile:663)
	at atv.d(SourceFile:619)
	at net.minecraft.client.main.Main.main(SourceFile:101)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
	at java.net.URLEncoder.encode(Unknown Source)
	at bcw.a(SourceFile:136)
	at bcw.a(SourceFile:123)
	at bcw.a(SourceFile:110)
	at fj.a(SourceFile:41)
	at co.b(SourceFile:354)
	at bcw.e(SourceFile:98)
	at bcy.c(SourceFile:71)

-- Affected screen --
Details:
	Screen name: bcy
Stacktrace:
	at atv.k(SourceFile:1173)
	at atv.S(SourceFile:663)
	at atv.d(SourceFile:619)
	at net.minecraft.client.main.Main.main(SourceFile:101)

-- System Details --
Details:
	Minecraft Version: 1.6.4
	Operating System: Windows XP (x86) version 5.1
	Java Version: 1.7.0_45, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
	Memory: 27205992 bytes (25 MB) / 71532544 bytes (68 MB) up to 518979584 bytes (494 MB)
	JVM Flags: 1 total; -Xmx512M
	AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	Launched Version: 1.6.4
	LWJGL: 2.9.0
	OpenGL: Intel(R) HD Graphics GL version 2.1.0 - Build 6.14.10.5258, Intel
	Is Modded: Probably not. Jar signature remains and client brand is untouched.
	Type: Client (map_client.txt)
	Resource Pack: Default
	Current Language: English (US)