Do not use MPN in the ExternalProductID container of AddItem, use the BrandMPN container in ProductListingDetails instead.

When enter a MPN in the ExternalProductID field, I recieve the error,

     <Errors>
          <ShortMessage>Input data is invalid.</ShortMessage>
          <LongMessage>Input data for tag &lt;ExternalProductIdentifierType&gt;
               is invalid or missing. Please check API documentation.</LongMessage>
          <ErrorCode>37</ErrorCode>
          <SeverityCode>Error</SeverityCode>
          <ErrorParameters ParamID=”0″>
               <Value>ExternalProductIdentifierType</Value>
          </ErrorParameters>
          <ErrorClassification>RequestError</ErrorClassification>
     </Errors>

Why am I getting this?