Backup error

[16:52:04] [Server thread/ERROR]: Couldn't process command: 'backup'
java.lang.NoSuchMethodError: aroma1997.core.util.ServerUtil.isPlayerAdmin(Ljava/lang/String;)Z
    at aroma1997.backup.BackupCommand.func_71519_b(BackupCommand.java:46) ~[BackupCommand.class:?]
    at net.minecraft.command.CommandHandler.func_71556_a(CommandHandler.java:51) [z.class:?]
    at net.minecraft.network.NetHandlerPlayServer.func_147361_d(NetHandlerPlayServer.java:734) [nh.class:?]
    at net.minecraft.network.NetHandlerPlayServer.func_147354_a(NetHandlerPlayServer.java:713) [nh.class:?]
    at net.minecraft.network.play.client.C01PacketChatMessage.func_148833_a(SourceFile:37) [ir.class:?]
    at net.minecraft.network.play.client.C01PacketChatMessage.func_148833_a(SourceFile:9) [ir.class:?]
    at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:212) [ej.class:?]
    at net.minecraft.network.NetworkSystem.func_151269_c(NetworkSystem.java:165) [nc.class:?]
    at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:659) [MinecraftServer.class:?]
    at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:335) [lt.class:?]
    at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:547) [MinecraftServer.class:?]
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:427) [MinecraftServer.class:?]
    at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685) [li.class:?]
[16:52:12] [Server thread/INFO]: An unknown error occurred while attempting to perform this command
[16:52:12] [Server thread/ERROR]: Couldn't process command: 'backup'
java.lang.NoSuchMethodError: aroma1997.core.util.ServerUtil.isPlayerAdmin(Ljava/lang/String;)Z
    at aroma1997.backup.BackupCommand.func_71519_b(BackupCommand.java:46) ~[BackupCommand.class:?]
    at net.minecraft.command.CommandHandler.func_71556_a(CommandHandler.java:51) [z.class:?]
    at net.minecraft.server.dedicated.DedicatedServer.func_71333_ah(DedicatedServer.java:371) [lt.class:?]
    at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:336) [lt.class:?]
    at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:547) [MinecraftServer.class:?]
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:427) [MinecraftServer.class:?]
    at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685) [li.class:?]