diff --git a/docs/howto/custom-model-fields.txt b/docs/howto/custom-model-fields.txt
index 1377a62c89..99c5904fe9 100644
--- a/docs/howto/custom-model-fields.txt
+++ b/docs/howto/custom-model-fields.txt
@@ -666,7 +666,7 @@ data storage anyway, we can reuse some existing conversion code::
 
         def value_to_string(self, obj):
             value = self._get_val_from_obj(obj)
-            return self.get_db_prep_value(value)
+            return self.get_prep_value(value)
 
 Some general advice
 --------------------