From Lua 5.3: assert() accepts any type of error object.

doc/extensions.html

@@ -373,6 +373,7 @@ LuaJIT supports some extensions from Lua 5.3:
 <li>Unicode escape <tt>'\u{XX...}'</tt> embeds the UTF-8 encoding in string literals.</li>
 <li>The argument table <tt>arg</tt> can be read (and modified) by <tt>LUA_INIT</tt> and <tt>-e</tt> chunks.</li>
 <li><tt>io.read()</tt> and <tt>file:read()</tt> accept formats with or without a leading <tt>*</tt>.</li>
+<li><tt>assert()</tt> accepts any type of error object.</li>
 <li><tt>table.move(a1, f, e, t [,a2])</tt>.</li>
 <li><tt>coroutine.isyieldable()</tt>.</li>
 <li>Lua/C API extensions: