ThreeShotMan33001

I have a problem about my game keep crashing because of my code. Code: package net.flexstudios.armorytests.items; import com.google.common.collect.Multimap; import net.minecraft.entity.ai.attributes.Attribute; import net.minecraft.entity.ai.attributes.AttributeModifier; import net.minecraft.entity.ai.attributes.Attributes; import net.minecraft.inventory.EquipmentSlotType; import net.minecraft.item.*; public class PoweredChestplateItem extends ArmorItem { public PoweredChestplateItem(IArmorMaterial materialIn, EquipmentSlotType slot, Properties builder) { super(materialIn, slot, builder); } @Override public Multimap<Attribute, AttributeModifier> getAttributeModifiers(EquipmentSlotType equipmentSlot) { Multimap<Attribute, AttributeModifier> result = super.getAttributeModifiers(equipmentSlot); result.put(Attributes.ATTACK_DAMAGE, new AttributeModifier(ATTACK_DAMAGE_MODIFIER, "Armor dmg", (double)4, AttributeModifier.Operation.ADDITION)); return result; } } A part of the crashing report: Description: Initializing game java.lang.UnsupportedOperationException: null at com.google.common.collect.ImmutableMultimap.put(ImmutableMultimap.java:427) ~[guava-21.0.jar:?] {} at net.flexstudios.armorytests.items.PoweredChestplateItem.func_111205_h(PoweredChestplateItem.java:19) ~[?:beta1.0.0] {re:classloading} at net.minecraftforge.common.extensions.IForgeItem.getAttributeModifiers(IForgeItem.java:77) ~[?:?] {re:classloading} at net.minecraft.item.ItemStack.func_111283_C(ItemStack.java:844) ~[?:?] {re:classloading} at net.minecraft.item.ItemStack.func_82840_a(ItemStack.java:614) ~[?:?] {re:classloading} at net.minecraft.client.Minecraft.lambda$populateSearchTreeManager$7(Minecraft.java:642) ~[?:?]