Paper/nms-patches/MerchantRecipe.patch

--- a/net/minecraft/server/MerchantRecipe.java
+++ b/net/minecraft/server/MerchantRecipe.java
@@ -1,5 +1,7 @@
 package net.minecraft.server;
 
+import org.bukkit.craftbukkit.inventory.CraftMerchantRecipe; // CraftBukkit
+
 public class MerchantRecipe {
 
     public ItemStack buyingItem1;
@@ -12,6 +14,18 @@
     private int demand;
     public float priceMultiplier;
     public int xp;
+    // CraftBukkit start
+    private CraftMerchantRecipe bukkitHandle;
+
+    public CraftMerchantRecipe asBukkit() {
+        return (bukkitHandle == null) ? bukkitHandle = new CraftMerchantRecipe(this) : bukkitHandle;
+    }
+
+    public MerchantRecipe(ItemStack itemstack, ItemStack itemstack1, ItemStack itemstack2, int uses, int maxUses, int experience, float priceMultiplier, CraftMerchantRecipe bukkit) {
+        this(itemstack, itemstack1, itemstack2, uses, maxUses, experience, priceMultiplier);
+        this.bukkitHandle = bukkit;
+    }
+    // CraftBukkit end
 
     public MerchantRecipe(NBTTagCompound nbttagcompound) {
         this.rewardExp = true;
@@ -73,6 +87,7 @@
 
     public ItemStack getBuyItem1() {
         int i = this.buyingItem1.getCount();
+        if (i <= 0) return ItemStack.a; // CraftBukkit - SPIGOT-5476
         ItemStack itemstack = this.buyingItem1.cloneItemStack();
         int j = Math.max(0, MathHelper.d((float) (i * this.demand) * this.priceMultiplier));