diff --git a/Source/objects.cpp b/Source/objects.cpp
index b489fa3d7..4f47f6b3f 100644
--- a/Source/objects.cpp
+++ b/Source/objects.cpp
@@ -2524,21 +2524,12 @@ void OperateShrineCostOfWisdom(Player &player, SpellID spellId, diablo_message m
 		}
 	}
 
-	int maxBase = player._pMaxManaBase;
-
-	if (maxBase < 0) {
-		// Fix bugged state; do not turn this into a "negative penalty" mana boost.
+	if (player._pMaxManaBase < 0)
 		player._pMaxManaBase = 0;
-		maxBase = 0;
-	}
-
-	const int penalty = maxBase / 10; // 10% of max base mana (>= 0)
 
-	player._pMaxManaBase -= penalty; // will remain >= 0
-	player._pManaBase -= penalty;    // may go negative, allowed
-	player._pMaxMana -= penalty;     // may go negative, allowed
-	player._pMana -= penalty;        // may go negative, allowed
+	const int penalty = player._pMaxManaBase / 10;
 
+	ModifyPlrManaCapacity(player, -penalty, true);
 	RedrawEverything();
 	InitDiabloMsg(message);
 }
diff --git a/Source/player.cpp b/Source/player.cpp
index 1b6f14096..f0dac04eb 100644
--- a/Source/player.cpp
+++ b/Source/player.cpp
@@ -3374,6 +3374,25 @@ void ModifyPlrVit(Player &player, int l)
 	}
 }
 
+void ModifyPlrManaCapacity(Player &player, int delta, bool shiftCurrent)
+{
+	constexpr int MinManaBase = 0;
+
+	const int newMaxBase = std::max(MinManaBase, player._pMaxManaBase + delta);
+	const int applied = newMaxBase - player._pMaxManaBase;
+
+	player._pMaxManaBase = newMaxBase;
+	player._pMaxMana += applied;
+
+	if (shiftCurrent) {
+		player._pManaBase += applied;
+		player._pMana += applied;
+	} else {
+		player._pManaBase = std::min(player._pManaBase, player._pMaxManaBase);
+		player._pMana = std::min(player._pMana, player._pMaxMana);
+	}
+}
+
 void SetPlayerHitPoints(Player &player, int val)
 {
 	player._pHitPoints = val;
diff --git a/Source/player.h b/Source/player.h
index 5cc99ca19..521766150 100644
--- a/Source/player.h
+++ b/Source/player.h
@@ -990,6 +990,7 @@ void ModifyPlrStr(Player &player, int l);
 void ModifyPlrMag(Player &player, int l);
 void ModifyPlrDex(Player &player, int l);
 void ModifyPlrVit(Player &player, int l);
+void ModifyPlrManaCapacity(Player &player, int delta, bool shiftCurrent);
 void SetPlayerHitPoints(Player &player, int val);
 void SetPlrStr(Player &player, int v);
 void SetPlrMag(Player &player, int v);