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

src/lib_base.c

@@ -42,13 +42,13 @@
 
 LJLIB_ASM(assert)		LJLIB_REC(.)
 {
-  GCstr *s;
   lj_lib_checkany(L, 1);
-  s = lj_lib_optstr(L, 2);
-  if (s)
-    lj_err_callermsg(L, strdata(s));
-  else
+  if (L->top == L->base+1)
     lj_err_caller(L, LJ_ERR_ASSERT);
+  else if (tvisstr(L->base+1) || tvisnumber(L->base+1))
+    lj_err_callermsg(L, strdata(lj_lib_checkstr(L, 2)));
+  else
+    lj_err_run(L);
   return FFH_UNREACHABLE;
 }