diff --git a/doc/api.rst b/doc/api.rst
index 47bb42c3..c3137466 100644
--- a/doc/api.rst
+++ b/doc/api.rst
@@ -120,6 +120,8 @@ specializations.
 
 .. doxygendefine:: FMT_STRING
 
+.. _udt:
+
 Formatting User-defined Types
 -----------------------------