Index: code/parse/lua.cpp
===================================================================
--- code/parse/lua.cpp	(revision 9923)
+++ code/parse/lua.cpp	(working copy)
@@ -4420,7 +4420,7 @@
 	if(idx < 0 || idx >= Num_weapon_types)
 		return ade_set_args(L, "i", -1);
 
-	return ade_set_args(L, "i", idx);
+	return ade_set_args(L, "i", idx + 1);
 }
 
 ADE_FUNC(isLaser, l_Weaponclass, NULL, "Return true if the weapon is a primary weapon (this includes Beams). This function is deprecated, use isPrimary instead.", "boolean", "true if the weapon is a primary, false otherwise")
@@ -6280,7 +6280,7 @@
 	if(idx < 0 || idx >= Num_ship_classes)
 		return ade_set_args(L, "i", -1);
 
-	return ade_set_args(L, "i", idx);
+	return ade_set_args(L, "i", idx + 1); // Lua is 1-based
 }
 
 //**********HANDLE: Debris
@@ -14212,7 +14212,13 @@
 	if(idx < 0) {
 		idx = atoi(name);
 
-		if(idx < 1 || idx > Num_weapon_types) {
+		// atoi is good enough here, 0 is invalid anyway
+		if (idx > 0)
+		{
+			idx--; // Lua --> C/C++
+		}
+		else
+		{
 			return ade_set_args(L, "o", l_Weaponclass.Set(-1));
 		}
 	}