Exception thrown creating new record for custom table

We created a custom table through the admin interface following the instructions at https://docs.xperience.io/k11/developing-websites/defining-website-content-structure/custom-tables/creating-custom-tables Now we are beginning to test some code that interacts with the table via the DataClassProvider. This is the code that tries to insert a new row, and as you can see except for changing the variable identifiers it is copied exactly from https://docs.xperience.io/api11/content-management/custom-table-data#Customtabledata-Addingdatarecordstoacustomtable:

string nonceClassName = "customtable.Nonce";
DataClassInfo nonceTable = DataClassInfoProvider.GetDataClassInfo(nonceClassName);
if (null != nonceTable)
{
    var newNonce = CustomTableItem.New(nonceClassName);
    newNonce.SetValue("NonceID", nonceId);
    newNonce.SetValue("NonceValue", nonceValue);
    nonceTable.Insert();
}

But when execution reaches this block IIS emits the following Exception information:

[CodeNameNotUniqueException: The custom table with code name 'customtable.Nonce' already exists.]
   CMS.DataEngine.AbstractInfoProvider`3.SetInfo(TInfo info) +871
   CMS.DataEngine.DataClassInfoProvider.SetInfo(DataClassInfo info) +347
   PaTTAN.Auth.OpenIdHelper.CreateNonce(Int32 byteCount) +433
...

Everything I've been able to find about CodeNameNotUniqueException describes situations that do not seem to apply here, mostly having to do with object caching on repeated calls. Can anyone suggest what we ought to be doing differently? I'd hate to have to write to the DB outside the API, but I do need to get this working fairly soon.