updateString error in PostgreSQL7.4 with JDBC

Hi,
I have a problem with PostgreSQL 7.4. (With the old one 7.3 no probllem.)

I create a connection and a statement :
        Class.forName("org.postgresql.Driver");
        conn = DriverManager.getConnection(
                        O_Constants.DB_CONNECTION_URL,
                        O_Constants.DB_CONNECTION_USERNAME,
                        O_Constants.DB_CONNECTION_PASSWORD);
        st = conn.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,
ResultSet.CONCUR_UPDATABLE);
        rs = st.executeQuery("select * ...";

Then:
    rs.updateString(20 , ...);

The error is:
org.postgresql.util.PSQLException: Cannot update the result set because
it is either before the start or after the end of the results.
        at
org.postgresql.jdbc2.AbstractJdbc2ResultSet.updateValue(AbstractJdbc2ResultSet.java:1595)
        at
org.postgresql.jdbc2.AbstractJdbc2ResultSet.updateString(AbstractJdbc2ResultSet.java:1193)
        at admin.ValidationRequest.doGet(ValidationRequest.java:43)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:740)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
        at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:247)
        at
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193)
        at
org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:256)
        at
org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:643)
        at
org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:480)
        at
org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:995)
        at
org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
        at
org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:643)
        at
org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:480)
        at
org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:995)
        at
org.apache.catalina.core.StandardContext.invoke(StandardContext.java:2422)
        at
org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:180)
        at
org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:643)
        at
org.apache.catalina.valves.ErrorDispatcherValve.invoke(ErrorDispatcherValve.java:171)
        at
org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:641)
        at
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:163)
        at
org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:641)
        at
org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:480)
        at
org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:995)
        at
org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:174)
        at
org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:643)
        at
org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:480)
        at
org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:995)
        at
org.apache.coyote.tomcat4.CoyoteAdapter.service(CoyoteAdapter.java:199)
        at
org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:324)
        at
org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:395)
        at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:673)
        at
org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:615)
        at
org.apache.jk.common.SocketConnection.runIt(ChannelSocket.java:786)
        at
org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:683)
        at java.lang.Thread.run(Thread.java:536)

The JDBC is:
    pg74.215.jdbc3.jar

If you have any comments, please help.
Thanks...