diff --git a/doc/Custom-types.md b/doc/Custom-types.md
index e1b1781b..172783ee 100644
--- a/doc/Custom-types.md
+++ b/doc/Custom-types.md
@@ -87,6 +87,7 @@ import NumberField from 'admin-config/lib/Field/NumberField';
 class AmountField extends NumberField {
     constructor(name) {
         super(name);
+        this._type = 'amount';
         this._currency = '$';
     }
     currency(currency) {