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