diff --git a/doc/lua.adoc b/doc/lua.adoc
index 67ed82c7e..5f447fe78 100644
--- a/doc/lua.adoc
+++ b/doc/lua.adoc
@@ -838,6 +838,11 @@ Example:
 
 Create a room region, which can be irregular; use the boundary <<_map_characters,map character>> to restrict the floodfilled area.
 
+If using the first form with a selection and "lit", the lit area will extend
+outward 1 space from the selection to attempt to accomodate adjacent walls,
+regardless of whether they are actually walls or not. If using "unlit", this
+will not happen.
+
 Example:
 
  des.region(selection, lit);
diff --git a/src/sp_lev.c b/src/sp_lev.c
index eecbf43d6..e88fc0a9b 100644
--- a/src/sp_lev.c
+++ b/src/sp_lev.c
@@ -6061,12 +6061,12 @@ lspo_region(lua_State *L)
     } else if (argc == 2) {
         /* region(selection, "lit"); */
         static const char *const lits[] = { "unlit", "lit", NULL };
-        struct selectionvar *sel = l_selection_check(L, 1);
+        struct selectionvar *orig = l_selection_check(L, 1),
+                            *sel = selection_clone(orig);
 
         rlit = luaL_checkoption(L, 2, "lit", lits);
 
         /*
-    TODO: adjust region size for wall, but only if lit
     TODO: lit=random
         */
         if (rlit)