Swapper/sumo error

[11:05:59] [Server thread/WARN]: [Swapper] Task #742 for Swapper v1.0 generated an exception
java.lang.NoClassDefFoundError: net/minecraft/server/v1_7_R1/Packet
	at me.krists.swapper.Arena$2.run(Arena.java:265) ~[?:?]
	at org.bukkit.craftbukkit.v1_7_R4.scheduler.CraftTask.run(CraftTask.java:53) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
	at org.bukkit.craftbukkit.v1_7_R4.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:349) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
	at net.minecraft.server.v1_7_R4.MinecraftServer.v(MinecraftServer.java:600) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
	at net.minecraft.server.v1_7_R4.DedicatedServer.v(DedicatedServer.java:258) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
	at net.minecraft.server.v1_7_R4.MinecraftServer.u(MinecraftServer.java:558) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
	at net.minecraft.server.v1_7_R4.MinecraftServer.run(MinecraftServer.java:469) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
	at net.minecraft.server.v1_7_R4.ThreadServerApplication.run(SourceFile:628) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
Caused by: java.lang.ClassNotFoundException: net.minecraft.server.v1_7_R1.Packet
	at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:67) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
	at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:62) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
	at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_25]
	at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_25]
	... 8 more