diff --git a/wadsrc/static/zscript/base.txt b/wadsrc/static/zscript/base.txt
index d6f7f318ea..726e46fccc 100644
--- a/wadsrc/static/zscript/base.txt
+++ b/wadsrc/static/zscript/base.txt
@@ -449,7 +449,7 @@ struct DropItem native
 	native readonly DropItem Next;
 	native readonly name Name;
 	native readonly int Probability;
-	native int Amount;
+	native readonly int Amount;
 }
 
 class SpotState : Object native
diff --git a/wadsrc/static/zscript/doom/bossbrain.txt b/wadsrc/static/zscript/doom/bossbrain.txt
index 5b87182c9a..fadc280e3d 100644
--- a/wadsrc/static/zscript/doom/bossbrain.txt
+++ b/wadsrc/static/zscript/doom/bossbrain.txt
@@ -318,11 +318,12 @@ extend class Actor
 			{
 				if (di.Name != 'None')
 				{
-					if (di.Amount < 0)
+					int amt = di.Amount;
+					if (amt < 0)
 					{
-						di.Amount = 1; // default value is -1, we need a positive value.
+						amt = 1; // default value is -1, we need a positive value.
 					}
-					n += di.Amount; // this is how we can weight the list.
+					n += amt; // this is how we can weight the list.
 				}
 			}
 			di = drop;
@@ -331,7 +332,12 @@ extend class Actor
 			{
 				if (di.Name != 'none')
 				{
-					n -= di.Amount; // logically, none of the -1 values have survived by now.
+					int amt = di.Amount;
+					if (amt < 0)
+					{
+						amt = 1;
+					}
+					n -= amt;
 				}
 				if ((di.Next != null) && (n >= 0))
 				{
diff --git a/wadsrc/static/zscript/shared/randomspawner.txt b/wadsrc/static/zscript/shared/randomspawner.txt
index b6e28cc371..7488fdd4d1 100644
--- a/wadsrc/static/zscript/shared/randomspawner.txt
+++ b/wadsrc/static/zscript/shared/randomspawner.txt
@@ -48,8 +48,9 @@ class RandomSpawner : Actor
 				{
 					if (!nomonsters || !IsMonster(di))
 					{
-						if (di.Amount < 0) di.Amount = 1; // default value is -1, we need a positive value.
-						n += di.Amount; // this is how we can weight the list.
+						int amt = di.Amount;
+						if (amt < 0) amt = 1; // default value is -1, we need a positive value.
+						n += amt; // this is how we can weight the list.
 					}
 					di = di.Next;
 				}
@@ -69,7 +70,9 @@ class RandomSpawner : Actor
 				if (di.Name != 'None' &&
 					(!nomonsters || !IsMonster(di)))
 				{
-					n -= di.Amount;
+					int amt = di.Amount;
+					if (amt < 0) amt = 1;
+					n -= amt;
 					if ((di.Next != null) && (n > -1))
 						di = di.Next;
 					else