diff --git a/api/src/main/java/org/geysermc/geyser/api/item/custom/CustomItemData.java b/api/src/main/java/org/geysermc/geyser/api/item/custom/CustomItemData.java
index bbf7e1f1c..0f80aae0a 100644
--- a/api/src/main/java/org/geysermc/geyser/api/item/custom/CustomItemData.java
+++ b/api/src/main/java/org/geysermc/geyser/api/item/custom/CustomItemData.java
@@ -125,8 +125,7 @@ public interface CustomItemData {
      *
      * @return the stack size of the item
      */
-    @NonNegative
-    int stackSize();
+    @NonNegative int stackSize();
 
     /**
      * Gets the max damage of the item.
@@ -147,7 +146,7 @@ public interface CustomItemData {
      *
      * @return the attack damage of the item
      */
-    int attackDamage();
+    @NonNegative int attackDamage();
 
     /**
      * Gets the armor type of the item.
@@ -166,7 +165,7 @@ public interface CustomItemData {
      *
      * @return the armor protection value of the item
      */
-    int protectionValue();
+    @NonNegative int protectionValue();
 
     /**
      * Gets if the item is a hat. This is used to determine if the item should be rendered on the player's head, and
@@ -231,11 +230,11 @@ public interface CustomItemData {
 
         Builder maxDamage(@NonNegative int maxDamage);
 
-        Builder attackDamage(int attackDamage);
+        Builder attackDamage(@NonNegative int attackDamage);
 
         Builder armorType(@Nullable String armorType);
 
-        Builder protectionValue(int protectionValue);
+        Builder protectionValue(@NonNegative int protectionValue);
 
         Builder hat(boolean isHat);
 
diff --git a/api/src/main/java/org/geysermc/geyser/api/item/custom/NonVanillaCustomItemData.java b/api/src/main/java/org/geysermc/geyser/api/item/custom/NonVanillaCustomItemData.java
index 9718b0653..51d6a984c 100644
--- a/api/src/main/java/org/geysermc/geyser/api/item/custom/NonVanillaCustomItemData.java
+++ b/api/src/main/java/org/geysermc/geyser/api/item/custom/NonVanillaCustomItemData.java
@@ -149,13 +149,13 @@ public interface NonVanillaCustomItemData extends CustomItemData {
         Builder maxDamage(@NonNegative int maxDamage);
 
         @Override
-        Builder attackDamage(int attackDamage);
+        Builder attackDamage(@NonNegative int attackDamage);
 
         @Override
         Builder armorType(@Nullable String armorType);
 
         @Override
-        Builder protectionValue(int protectionValue);
+        Builder protectionValue(@NonNegative int protectionValue);
 
         @Override
         Builder hat(boolean isHat);
diff --git a/core/src/main/java/org/geysermc/geyser/item/GeyserCustomItemData.java b/core/src/main/java/org/geysermc/geyser/item/GeyserCustomItemData.java
index b43b921c5..6fb020729 100644
--- a/core/src/main/java/org/geysermc/geyser/item/GeyserCustomItemData.java
+++ b/core/src/main/java/org/geysermc/geyser/item/GeyserCustomItemData.java
@@ -27,6 +27,8 @@ package org.geysermc.geyser.item;
 
 import lombok.EqualsAndHashCode;
 import lombok.ToString;
+import org.checkerframework.checker.index.qual.NonNegative;
+import org.checkerframework.checker.index.qual.Positive;
 import org.checkerframework.checker.nullness.qual.NonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 import org.geysermc.geyser.api.item.custom.CustomItemData;
@@ -274,7 +276,7 @@ public class GeyserCustomItemData implements CustomItemData {
         }
 
         @Override
-        public Builder stackSize(int stackSize) {
+        public Builder stackSize(@Positive int stackSize) {
             if (stackSize < 1) {
                 throw new IllegalArgumentException("Stack size cannot be below 1 (" + stackSize + " was given)");
             } else if (stackSize > 1) {
@@ -290,7 +292,7 @@ public class GeyserCustomItemData implements CustomItemData {
         }
 
         @Override
-        public Builder maxDamage(int maxDamage) {
+        public Builder maxDamage(@NonNegative int maxDamage) {
             if (maxDamage < 0) {
                 throw new IllegalArgumentException("Max damage cannot be below 0 (" + maxDamage + " was given)");
             } else if (maxDamage > 0) {
@@ -306,7 +308,10 @@ public class GeyserCustomItemData implements CustomItemData {
         }
 
         @Override
-        public Builder attackDamage(int attackDamage) {
+        public Builder attackDamage(@NonNegative int attackDamage) {
+            if (attackDamage < 0) {
+                throw new IllegalArgumentException("Protection value cannot be below 0 (" + attackDamage + " was given)");
+            }
             this.attackDamage = attackDamage;
             return this;
         }
@@ -318,7 +323,10 @@ public class GeyserCustomItemData implements CustomItemData {
         }
 
         @Override
-        public Builder protectionValue(int protectionValue) {
+        public Builder protectionValue(@NonNegative int protectionValue) {
+            if (protectionValue < 0) {
+                throw new IllegalArgumentException("Protection value cannot be below 0 (" + protectionValue + " was given)");
+            }
             this.protectionValue = protectionValue;
             return this;
         }
diff --git a/core/src/main/java/org/geysermc/geyser/item/GeyserNonVanillaCustomItemData.java b/core/src/main/java/org/geysermc/geyser/item/GeyserNonVanillaCustomItemData.java
index c5ac4f68e..5ae41479c 100644
--- a/core/src/main/java/org/geysermc/geyser/item/GeyserNonVanillaCustomItemData.java
+++ b/core/src/main/java/org/geysermc/geyser/item/GeyserNonVanillaCustomItemData.java
@@ -27,6 +27,8 @@ package org.geysermc.geyser.item;
 
 import lombok.EqualsAndHashCode;
 import lombok.ToString;
+import org.checkerframework.checker.index.qual.NonNegative;
+import org.checkerframework.checker.index.qual.Positive;
 import org.checkerframework.checker.nullness.qual.NonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 import org.geysermc.geyser.api.item.custom.CustomItemOptions;
@@ -167,17 +169,17 @@ public final class GeyserNonVanillaCustomItemData extends GeyserCustomItemData i
         }
 
         @Override
-        public Builder stackSize(int stackSize) {
+        public Builder stackSize(@Positive int stackSize) {
             return (Builder) super.stackSize(stackSize);
         }
 
         @Override
-        public Builder maxDamage(int maxDamage) {
+        public Builder maxDamage(@NonNegative int maxDamage) {
             return (Builder) super.maxDamage(maxDamage);
         }
 
         @Override
-        public Builder attackDamage(int attackDamage) {
+        public Builder attackDamage(@NonNegative int attackDamage) {
             return (Builder) super.attackDamage(attackDamage);
         }
 
@@ -187,7 +189,7 @@ public final class GeyserNonVanillaCustomItemData extends GeyserCustomItemData i
         }
 
         @Override
-        public Builder protectionValue(int protectionValue) {
+        public Builder protectionValue(@NonNegative int protectionValue) {
             return (Builder) super.protectionValue(protectionValue);
         }