Detail in the manual

Function 'lua_toclose' can raise a non-memory error (but not a
memory error).

manual/manual.of

@@ -4416,7 +4416,7 @@ otherwise, returns @id{NULL}.
 }
 
 @APIEntry{void lua_toclose (lua_State *L, int index);|
-@apii{0,0,m}
+@apii{0,0,v}
 
 Marks the given index in the stack as a
 to-be-closed slot @see{to-be-closed}.
@@ -4433,6 +4433,9 @@ A slot marked as to-be-closed should not be removed from the stack
 by any other function in the API except @Lid{lua_settop} or @Lid{lua_pop},
 unless previously deactivated by @Lid{lua_closeslot}.
 
+This function raises an error if the value at the given slot
+neither has a @idx{__close} metamethod nor is a false value.
+
 This function should not be called for an index
 that is equal to or below an active to-be-closed slot.