MCRegion > Anvil doesn't work with old maps

Client> 2013-07-05 15:47:49 [SERVER] [INFO] Starting integrated minecraft server version 1.6.1
Client> 2013-07-05 15:47:49 [SERVER] [INFO] Generating keypair
Client> 2013-07-05 15:47:49 [SERVER] [INFO] Converting map!
Client> 2013-07-05 15:47:49 [SERVER] [INFO] Scanning folders...
Client> 2013-07-05 15:47:49 [SERVER] [INFO] Total conversion count is 4
Client> u: Reading NBT data
Client> 	at bx.f(SourceFile:140)
Client> 	at adu.a(SourceFile:30)
Client> 	at alg.a(SourceFile:215)
Client> 	at alg.a(SourceFile:186)
Client> 	at alg.a(SourceFile:145)
Client> 	at net.minecraft.server.MinecraftServer.a(SourceFile:113)
Client> 	at bko.a(SourceFile:61)
Client> 	at bko.d(SourceFile:106)
Client> 	at net.minecraft.server.MinecraftServer.run(SourceFile:314)
Client> 	at hf.run(SourceFile:582)
Client> Caused by: java.lang.ClassCastException: ce cannot be cast to cg
Client> 	at bx.f(SourceFile:138)
Client> 	... 9 more
Client> 2013-07-05 15:47:49 [SERVER] [SEVERE] Encountered an unexpected exception u
Client> u: Reading NBT data
Client> 	at bx.f(SourceFile:140)
Client> 	at adu.a(SourceFile:30)
Client> 	at alg.a(SourceFile:215)
Client> 	at alg.a(SourceFile:186)
Client> 	at alg.a(SourceFile:145)
Client> 	at net.minecraft.server.MinecraftServer.a(SourceFile:113)
Client> 	at bko.a(SourceFile:61)
Client> 	at bko.d(SourceFile:106)
Client> 	at net.minecraft.server.MinecraftServer.run(SourceFile:314)
Client> 	at hf.run(SourceFile:582)
Client> Caused by: java.lang.ClassCastException: ce cannot be cast to cg
Client> 	at bx.f(SourceFile:138)
Client> 	... 9 more

http://puu.sh/3wRi3.zip is a world that cannot be converted.

Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

Same problem in 1.6.2.

Duplicate of MC-5446

Duplicate of MC-5672, corrupted world.