Exception when setting OracleParameter to DbType.DateTimeOffset

I have the following code:

Oracle.ManagedDataAccess.Client.OracleParameter p;
// create p
p.DbType = DbType.DateTimeOffset

This is throwing:

System.ArgumentOutOfRangeException HResult=0x80131502
Message=Specified argument was out of the range of valid values.
Source=Oracle.ManagedDataAccess StackTrace: at Oracle.ManagedDataAccess.Client.OracleParameter.set_DbType(DbType value) at WindwardReportsDrivers.net.windward.datasource.ado.AdoQueryHelper.setParameter(IDbCommand cmd, Int32 varNum, Object item) in C:\git\Jen_2\engine\DotNetEngine\WindwardReportsDrivers\net\windward\datasource\ado\AdoQueryHelper.cs:line 522

Any idea what/why this is happening?