update index in Solr, error: required field in SolrSchema not found in DataConfig

Question

I'm trying to update my index, but I keep on getting the error:

org.apache.solr.handler.dataimport.DataImporter verifyWithSchema INFO: UPC is a required field in SolrSchema . But not found in DataConfigfound in DataConfig

I can't figure out why it's complainting, since:

1. the first time I ran the import, it worked fine, and the only thing I changed was add a few fields (columns) to schema.xml

2. the table I am querying indeed has a UPC column. Here is what my data-config.xml looks like:

   <dataConfig>
     <dataSource type="JdbcDataSource" 
         driver="com.mysql.jdbc.Driver"
         url="jdbc:mysql://localhost:3306/product" 
         user="root" 
         password="some_password"/>
     <document>
       <entity name="product" 
          query="select * from productdetails">
       </entity>
     </document>
    </dataConfig>
   

But again, the interesting part is that the import worked a second ago, but fails on re-import. I'm hoping somebody has had this problem before. If not, maybe someone can suggest other things to check for?