BukkitError

[SEVERE] Could not call method 'public static org.bukkit.inventory.ItemStack org.bukkit.inventory.ItemStack.deserialize(java.util.Map)' of class org.bukkit.inventory.ItemStack for deserialization
java.lang.NullPointerException
        at org.bukkit.inventory.ItemStack.<init>(ItemStack.java:94)
        at org.bukkit.inventory.ItemStack.deserialize(ItemStack.java:519)
        at sun.reflect.GeneratedMethodAccessor8.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at org.bukkit.configuration.serialization.ConfigurationSerialization.deserializeViaMethod(ConfigurationSerialization.java:73)
        at org.bukkit.configuration.serialization.ConfigurationSerialization.deserialize(ConfigurationSerialization.java:113)
        at org.bukkit.configuration.serialization.ConfigurationSerialization.deserializeObject(ConfigurationSerialization.java:187)
        at org.bukkit.configuration.file.YamlConstructor$ConstructCustomObject.construct(YamlConstructor.java:35)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructObject(BaseConstructor.java:182)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructMapping2ndStep(BaseConstructor.java:325)
        at org.yaml.snakeyaml.constructor.SafeConstructor.constructMapping2ndStep(SafeConstructor.java:145)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructMapping(BaseConstructor.java:306)
        at org.yaml.snakeyaml.constructor.SafeConstructor$ConstructYamlMap.construct(SafeConstructor.java:461)
        at org.bukkit.configuration.file.YamlConstructor$ConstructCustomObject.construct(YamlConstructor.java:26)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructObject(BaseConstructor.java:182)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructMapping2ndStep(BaseConstructor.java:325)
        at org.yaml.snakeyaml.constructor.SafeConstructor.constructMapping2ndStep(SafeConstructor.java:145)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructMapping(BaseConstructor.java:306)
        at org.yaml.snakeyaml.constructor.SafeConstructor$ConstructYamlMap.construct(SafeConstructor.java:461)
        at org.bukkit.configuration.file.YamlConstructor$ConstructCustomObject.construct(YamlConstructor.java:26)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructObject(BaseConstructor.java:182)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructMapping2ndStep(BaseConstructor.java:325)
        at org.yaml.snakeyaml.constructor.SafeConstructor.constructMapping2ndStep(SafeConstructor.java:145)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructMapping(BaseConstructor.java:306)
        at org.yaml.snakeyaml.constructor.SafeConstructor$ConstructYamlMap.construct(SafeConstructor.java:461)
        at org.bukkit.configuration.file.YamlConstructor$ConstructCustomObject.construct(YamlConstructor.java:26)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructObject(BaseConstructor.java:182)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructMapping2ndStep(BaseConstructor.java:325)
        at org.yaml.snakeyaml.constructor.SafeConstructor.constructMapping2ndStep(SafeConstructor.java:145)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructMapping(BaseConstructor.java:306)
        at org.yaml.snakeyaml.constructor.SafeConstructor$ConstructYamlMap.construct(SafeConstructor.java:461)
        at org.bukkit.configuration.file.YamlConstructor$ConstructCustomObject.construct(YamlConstructor.java:26)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructObject(BaseConstructor.java:182)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructMapping2ndStep(BaseConstructor.java:325)
        at org.yaml.snakeyaml.constructor.SafeConstructor.constructMapping2ndStep(SafeConstructor.java:145)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructMapping(BaseConstructor.java:306)
        at org.yaml.snakeyaml.constructor.SafeConstructor$ConstructYamlMap.construct(SafeConstructor.java:461)
        at org.bukkit.configuration.file.YamlConstructor$ConstructCustomObject.construct(YamlConstructor.java:26)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructObject(BaseConstructor.java:182)
        at org.yaml.snakeyaml.constructor.BaseConstructor.constructDocument(BaseConstructor.java:141)
        at org.yaml.snakeyaml.constructor.BaseConstructor.getSingleData(BaseConstructor.java:127)
        at org.yaml.snakeyaml.Yaml.loadFromReader(Yaml.java:481)
        at org.yaml.snakeyaml.Yaml.load(Yaml.java:400)
        at org.bukkit.configuration.file.YamlConfiguration.loadFromString(YamlConfiguration.java:53)
        at com.nisovin.shopkeepers.ShopkeepersPlugin.load(ShopkeepersPlugin.java:831)
        at com.nisovin.shopkeepers.ShopkeepersPlugin.onEnable(ShopkeepersPlugin.java:127)
        at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:217)
        at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:467)
        at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:383)
        at org.bukkit.craftbukkit.v1_6_R3.CraftServer.loadPlugin(CraftServer.java:277)
        at org.bukkit.craftbukkit.v1_6_R3.CraftServer.enablePlugins(CraftServer.java:259)
        at net.minecraft.server.MinecraftServer.func_71243_i(MinecraftServer.java:525)
        at net.minecraft.server.MinecraftServer.func_71222_d(MinecraftServer.java:489)
        at net.minecraft.server.MinecraftServer.func_71247_a(MinecraftServer.java:432)
        at net.minecraft.server.dedicated.DedicatedServer.func_71197_b(DedicatedServer.java:224)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:634)
        at net.minecraft.server.ThreadMinecraftServer.run(ThreadMinecraftServer.java:16)