Index: ship/ship.cpp
===================================================================
--- ship/ship.cpp	(revision 9638)
+++ ship/ship.cpp	(working copy)
@@ -15244,10 +15251,14 @@
 int get_max_ammo_count_for_bank(int ship_class, int bank, int ammo_type)
 {
 	float capacity, size;
-
-	capacity = (float) Ship_info[ship_class].secondary_bank_ammo_capacity[bank];
-	size = (float) Weapon_info[ammo_type].cargo_size;
-	return (int) (capacity / size);
+    
+	if (ship_class < 0 || bank < 0 || ammo_type < 0) {
+		return 0;
+	} else {
+		capacity = (float) Ship_info[ship_class].secondary_bank_ammo_capacity[bank];
+		size = (float) Weapon_info[ammo_type].cargo_size;
+		return (int) (capacity / size);
+	}
 }
 
 /**