diff --git a/source/glest_game/types/upgrade_type.cpp b/source/glest_game/types/upgrade_type.cpp
index f8bedb900..b2ab4c784 100644
--- a/source/glest_game/types/upgrade_type.cpp
+++ b/source/glest_game/types/upgrade_type.cpp
@@ -249,13 +249,21 @@ void TotalUpgrade::reset() {
 
 void TotalUpgrade::sum(const UpgradeTypeBase *ut) {
 	maxHp+= ut->getMaxHp();
+	maxHp = max(0,maxHp);
 	maxEp+= ut->getMaxEp();
+	maxEp = max(0,maxEp);
 	sight+= ut->getSight();
+	sight = max(0,sight);
 	armor+= ut->getArmor();
+	armor = max(0,armor);
 	attackStrength+= ut->getAttackStrength();
+	attackStrength = max(0,attackStrength);
 	attackRange+= ut->getAttackRange();
+	attackRange = max(0,attackRange);
 	moveSpeed+= ut->getMoveSpeed();
+	moveSpeed = max(0,moveSpeed);
     prodSpeed+= ut->getProdSpeed();
+    prodSpeed = max(0,prodSpeed);
 }
 
 void TotalUpgrade::incLevel(const UnitType *ut) {