Null bind variable in where clause

It looks like you can not use a bind variable when trying to match null
in a where clause.  Is there any way to get the desired effect without
having to remove the bind variable and rewrite the query to "WHERE b IS
null"?

import java.sql.*;

public class a {

    public static void main(String args[]) throws Exception {
        String sql = "CREATE TABLE t(a int, b int)";
        Class.forName("org.postgresql.Driver");
        Connection conn = DriverManager.getConnection(
                "jdbc:postgresql://10.20.1.70:5432/firm", "gaiam", "");
        try {
            Statement stmt = conn.createStatement();
            stmt.executeUpdate(sql);
            sql = "INSERT INTO t(a,b) VALUES (3,null)";
            stmt.executeUpdate(sql);
            stmt.close();
        } catch (SQLException sqle) {
            sqle.printStackTrace();
        }

        sql = "UPDATE t SET a = ? WHERE b = ?";
        PreparedStatement pstmt = conn.prepareStatement(sql);
        pstmt.setInt(1, 4);
        pstmt.setObject(2, null);
        int x = pstmt.executeUpdate();
        System.out.println(x);
        pstmt.close();
        Statement stmt = conn.createStatement();
        stmt.executeUpdate("DROP TABLE t");
        stmt.close();
        conn.close();
    }
}

Will print 0 instead of 1

-Chris