سلام.
توی لینک های زیر راه حل هایی ارائه شده. اگه به نتیجه نرسیدین بگین تا فارسی توضیح بدم
http://www.daniweb.com/software-development/vbnet/threads/341393/conversion-from-type-dbnull-to-type-integer-is-not-valid-problem
http://www.vbforums.com/showthread.php?517505-RESOLVED-Conversion-from-type-DBNull-to-type-Integer-is-not-valid.
http://www.codeproject.com/Questions/333840/conversion-from-type-dbnull-to-type-integer-is-not
http://www.codeproject.com/Questions/255517/Conversion-from-type-DBNull-to-type-Integer-is-not
http://www.dotnetnuke.com/Resources/Forums/forumid/111/threadid/64867/scope/posts.aspx
http://www.dreamincode.net/forums/topic/265232-conversion-from-type-dbnull-to-type-integer-is-not-valid/
http://social.msdn.microsoft.com/Forums/en-US/vbgeneral/thread/57f2070f-b2dc-47a4-b8ad-b3a650104f0e
یاعلی