diff --git a/src/calibre/utils/formatter_functions.py b/src/calibre/utils/formatter_functions.py
index 005e1a6932..4e4a2c22a8 100644
--- a/src/calibre/utils/formatter_functions.py
+++ b/src/calibre/utils/formatter_functions.py
@@ -295,8 +295,8 @@ class FormatterFunction:
 
 class BuiltinFormatterFunction(FormatterFunction):
 
-    def __init__(self):
-        formatter_functions().register_builtin(self)
+    def __init_subclass__(cls):
+        formatter_functions().register_builtin(cls())
 
 
 class BuiltinStrcmp(BuiltinFormatterFunction):
@@ -3843,12 +3843,6 @@ returns `Foo, book 3 of 5`
         raise ValueError(_('This function cannot be called directly. It is built into the formatter'))
 
 
-# Registering builtins
-for obj in list(globals().values()):
-    if isinstance(obj, type) and issubclass(obj, BuiltinFormatterFunction) and obj is not BuiltinFormatterFunction:
-        obj()
-
-
 class FormatterUserFunction(FormatterFunction):
 
     def __init__(self, name, doc, arg_count, program_text, object_type):