Interesting error thrown on SQL with text field that contains a date

I have a table that carries date values as strings in varchar columns. A select statement like this:

select * from vwPunches v,tbltclocs l
where v.clockname = l.clockname and location = 'GARAGE' and
srtDate between '2012-06-06' and '2012-06-21'

Works fine. If I parameterize it with a statement like this:

String sql = select * from vwPunches v,tbltclocs l
where v.clockname = l.clockname and location = ? and
srtDate between ? and ?

And then execute it like so:

ps1 = conn.prepareStatement(sqlSelectPunchErrors);
ps1.setString(1, "GARAGE");
ps1.setString(2, "2012-06-06");
ps1.setString(3, "2012-06-21");

The second setString ( ps1.setString(2, "2012-06-06");) throws this error:

java.sql.SQLException: Data type mismatch. (class java.lang.NumberFormatException)
at com.ibm.as400.access.JDError.throwSQLException(JDError.java:521)
at com.ibm.as400.access.SQLVarcharForBitData.set(SQLVarcharForBitData.java:100)
at com.ibm.as400.access.AS400JDBCPreparedStatement.setValue(AS400JDBCPreparedStatement.java:2817)
at com.ibm.as400.access.AS400JDBCPreparedStatement.setString(AS400JDBCPreparedStatement.java:2470)
at org.apache.commons.dbcp.DelegatingPreparedStatement.setString(DelegatingPreparedStatement.java:132)

srtDate is defined as a varChar(10) not a number.....

Seems like the JT400 driver is attempting to convert the date to/from bit data (I think). The column using the between operator is a varChar and the parameter values are Strings. I don't see how that is causing the Exception to be thrown.

A bug perhaps?