i am trying to create a foreign key in my table. But when i executes my query it shows me error 150

Error Code : 1005 Can't create table '.\vts#sql-6ec_1.frm' (errno: 150) (0 ms taken)

My Queries are

Query to create a foreign Key

alter table `vts`.`tblguardian` add constraint `FK_tblguardian` FOREIGN KEY (`GuardianPickPointId`) REFERENCES `tblpickpoint` (`PickPointId`)

EDIT:

Now I am trying to drop this constraint But it fails again and shows me same error as it was giving when i was trying to create foreign key.

alter table `vts`.`tblguardian` drop index `FK_tblguardian`

Primary Key table

CREATE TABLE `tblpickpoint` (                                                           
                `PickPointId` int(4) NOT NULL auto_increment,                                         
                `PickPointName` varchar(500) default NULL,                                            
                `PickPointLabel` varchar(500) default NULL,                                           
                `PickPointLatLong` varchar(100) NOT NULL,                                             
                PRIMARY KEY  (`PickPointId`)                                                          
              ) ENGINE=InnoDB DEFAULT CHARSET=latin1 CHECKSUM=1 DELAY_KEY_WRITE=1 ROW_FORMAT=DYNAMIC  

Foreign Key Table

CREATE TABLE `tblguardian` (                           
               `GuardianId` int(4) NOT NULL auto_increment,         
               `GuardianName` varchar(500) default NULL,            
               `GuardianAddress` varchar(500) default NULL,         
               `GuardianMobilePrimary` varchar(15) NOT NULL,        
               `GuardianMobileSecondary` varchar(15) default NULL,  
               `GuardianPickPointId` int(4) default NULL,     
               PRIMARY KEY  (`GuardianId`)                          
             ) ENGINE=InnoDB DEFAULT CHARSET=latin1