PaperMC/paper-server/nms-patches/net/minecraft/world/item/trading/MerchantRecipe.patch

--- a/net/minecraft/world/item/trading/MerchantRecipe.java
+++ b/net/minecraft/world/item/trading/MerchantRecipe.java
@@ -5,6 +5,8 @@
 import net.minecraft.util.MathHelper;
 import net.minecraft.world.item.ItemStack;
 
+import org.bukkit.craftbukkit.inventory.CraftMerchantRecipe; // CraftBukkit
+
 public class MerchantRecipe {
 
     public ItemStack baseCostA;
@@ -17,6 +19,18 @@
     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;
@@ -78,6 +92,7 @@
 
     public ItemStack getBuyItem1() {
         int i = this.baseCostA.getCount();
+        if (i <= 0) return ItemStack.EMPTY; // CraftBukkit - SPIGOT-5476
         ItemStack itemstack = this.baseCostA.cloneItemStack();
         int j = Math.max(0, MathHelper.d((float) (i * this.demand) * this.priceMultiplier));