diff --git a/src/Services/AchievementService.cs b/src/Services/AchievementService.cs
index d4f4449..e7079f3 100644
--- a/src/Services/AchievementService.cs
+++ b/src/Services/AchievementService.cs
@@ -54,6 +54,11 @@ namespace sodoff.Services {
         public AchievementReward? AddDragonAchievementPoints(Dragon dragon, int? value) {
             dragon.PetXP = (dragon.PetXP ?? 0) + (value ?? 0);
 
+            if (dragon.PetXP < 0) {
+                dragon.PetXP = int.MaxValue;
+                value = 0;
+            }
+
             return new AchievementReward{
                 // NOTE: RewardID and EntityTypeID are not used by client
                 EntityID = Guid.Parse(dragon.EntityId),
@@ -76,6 +81,11 @@ namespace sodoff.Services {
                 }
                 xpPoints.Value += value ?? 0;
 
+                if (xpPoints.Value < 0) {
+                    xpPoints.Value = int.MaxValue;
+                    value = 0;
+                }
+
                 return new AchievementReward{
                     EntityID = Guid.Parse(viking.Id),
                     PointTypeID = type,