diff --git a/docs/user-defined-type-functions.md b/docs/user-defined-type-functions.md
index dba3c65..55a9186 100644
--- a/docs/user-defined-type-functions.md
+++ b/docs/user-defined-type-functions.md
@@ -50,7 +50,9 @@ For the first iteration, the body of a type function will be sandboxed, and its
 - global functions: `assert`, `error`, `next`, `print`, `rawequal`, `select`, `tonumber`, `tostring`, `type`, `typeof`, `ipairs`, `pairs`, `unpack`
 - math library
 - table library
+- string library
 - bit32 library
+- utf8 library
 - buffer library
 
 There is also a problem of infinitely running type functions. For example, reducing this type function will halt analysis until the VM stack overflows: