Server Crash After a few Seconds

The server will start and run normally but about 10 seconds in will crash with this error:

16:30:19] [Server thread/ERROR]: Encountered an unexpected exception
u: Exception ticking world
at net.minecraft.server.MinecraftServer.x(SourceFile:582) ~inecraft_server.jar:?]
at pd.x(SourceFile:285) ~inecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.w(SourceFile:510) ~inecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:426) inecraft_server.jar:?]
at java.lang.Thread.run(Unknown Source) [?:1.7.0_01]
Caused by: java.lang.IllegalArgumentException: Cannot get property bcl{name=age, clazz=class java.lang.Integer, values=[0, 1, 2, 3, 4, 5, 6, 7]} as it does not exist!
at bbw.b(SourceFile:91) ~inecraft_server.jar:?]
at ayt.b(SourceFile:127) ~inecraft_server.jar:?]
at ayt.c(SourceFile:69) ~inecraft_server.jar:?]
at ary.b(SourceFile:416) ~inecraft_server.jar:?]
at qg.h(SourceFile:373) ~inecraft_server.jar:?]
at qg.c(SourceFile:199) ~inecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.x(SourceFile:578) ~inecraft_server.jar:?]