MDB2 Error: null value violates not-null constraint

User Info
_doQuery: [Error message: Could not execute statement]
[Last executed query: INSERT INTO mb_usergroups (groupid,userid,joined) VALUES 
				(7,
				 NULL,
				 NOW())]
[Native code: 1048]
[Native message: Column 'userid' cannot be null]


Backtrace:
12: (0)                       ResourceManager->ErrorHandler(MDB2_Error)
11: PEAR.php(912)             call_user_func(array, MDB2_Error)
10: MDB2.php(973)             PEAR_Error->PEAR_Error("MDB2 Error", -29, 16, array, "_doQuery: ")
 9: PEAR.php(574)             MDB2_Error->MDB2_Error(-29, 16, array, "_doQuery: ")
 8: MDB2.php(1442)            PEAR->raiseError(NULL, -29, NULL, NULL, "_doQuery: ", "MDB2_Error", TRUE)
 7: mysql.php(607)            MDB2_Driver_Common->raiseError(NULL, NULL, NULL, "Could not ", "_doQuery")
 6: MDB2.php(2477)            MDB2_Driver_mysql->_doQuery("INSERT INT", TRUE, resource, "mybuffed_e")
 5: mbgroup.inc.php(1010)     MDB2_Driver_Common->exec("INSERT INT")
 4: mbgroup.inc.php(377)      Mybuffed_MbGroup->joinGroup(7, "")
 3: pagecontent.inc.php(294)  Mybuffed_MbGroup->getContent()
 2: getbuffed_pagehandler.inc.php(90) BuffedPageContent->makeContent()
 1: index.php(120)            require_once("/var/www/w")