SQLBindParameter怎么绑定NULL值

SQLBindParameter怎样绑定NULL值
对于表中的时间类型，当为空时，直接插入空字符串时会报错提示时间格式不正确，在SQL语句里能插入NULL值，我在C里面用SQLBindParameter该怎样绑定NULL值，写到时间类型的字段里？
SQL="INSERT CTEST (CLT_TIME) VALUES(?)"
parameter="2008-11-06 14:45:33.000000";

                 cliRC = SQLBindParameter(hstmt,                          
                             i,                              
                             SQL_PARAM_INPUT,                
                             SQL_C_CHAR,                     
                             SQL_CHAR,                       
                             26,
                             0,                              
                             parameter,                     
                             0,                              
                             NULL);        
当时间有值时，是可以插入的，当为空时，就不能插入，提示An invalid datetime format was detected; that is, an invalid string representation or value was specified. SQLSTATE=22007
我该怎么做？
谢谢
[解决办法]
Just Try:
------------------------
   cliRC = SQLBindParameter(hstmt,                          
                            i,                              

                            SQL_PARAM_INPUT,                
                            SQL_C_CHAR,                    
                            SQL_CHAR,                      
                            26, 
                            0,                              
                            SQL_NULL_DATA,        /* use this for test */            
                            0,                              
                            NULL);