KeyNotFoundException problem

Question

morning guys...hope you all in well always..i have problem in apriori project for c#....for simplify, the apriori algorithm is about data mining...you can check the code in this link http://www.codeproject.com/Articles/70371/Apriori-Algorithm .... i just wanna to test his tool for a number, but i have a problem there...if i used alphabet, the process work, but if i use digit, its result KeyNotFoundException...below are some pictures that relate with my problem....i hopu you guys can help me...thank you for spending your time read my problem (^_^)