json parse, then convert

index.html

@@ -663,14 +663,31 @@ <h2>
  <var>paymentMethod</var>.<a data-lt="PaymentMethodData.data">data</a>
  into a string. Rethrow any exceptions.
  </li>
- <li>If required by the specification that defines the
- <var>paymentMethod</var>.<a>supportedMethods</a>,
- <a data-lt="converting">convert</a>
- <var>paymentMethod</var>.<a>data</a> to an IDL value of the
- type specified there (e.g., a <a data-cite=
- "payment-method-basic-card#dom-basiccardrequest">BasicCardRequest</a>
- in the case of [[?payment-method-basic-card]]). Rethrow any
- exceptions.
+ <li>If <var>serializedData</var> is not null, and if required
+ by the specification that defines the
+ <var>paymentMethod</var>.<a>supportedMethods</a>:
+ <ol>
+ <li>Let <var>object</var> be the result of
+ <a data-cite="ECMASCRIPT#sec-json.parse">JSON-parsing</a>
+ <var>serializedData</var>.
+ </li>
+ <li>
+ <p>
+ <a data-lt="converting">Convert</a> <var>object</var>
+ to an IDL value of the type specified by the
+ specification that defines the
+ <var>paymentMethod</var>.<a>supportedMethods</a>
+ (e.g., a <a data-cite=
+ "payment-method-basic-card#dom-basiccardrequest">BasicCardRequest</a>
+ in the case of [[?payment-method-basic-card]]).
+ Rethrow any exceptions.
+ </p>
+ <p class="note">
+ This step assures that any IDL type conversion errors
+ are caught as early as possible.
+ </p>
+ </li>
+ </ol>
  </li>
  <li>Add the tuple (<var>paymentMethod</var>.<a data-lt=
  "PaymentMethodData.supportedMethods">supportedMethods</a>,